Nonvolatile semiconductor memory device having electrically and collectively erasable characteristics

ABSTRACT

A semiconductor memory device has 2 n  word lines, a plurality of bit lines, a plurality of nonvolatile memory cells disposed at each intersection of the word lines and the bit lines, a write circuit for writing data to a memory cell located at an intersection of selected ones of the word lines and the bit lines, and a sense amplifier for reading data out of the memory cells. Further, the semiconductor memory device comprises a first unit for simultaneously selecting a block of 2 m  (n&gt;m) word lines among the 2 n  word lines, and a second unit for not selecting a block of 2 k  (m&gt;k) word lines among the 2 m  word lines. The second unit does not select the block of 2 k  word lines, and selects a block of 2 k  word lines prepared outside the 2 n  word lines when any one of the 2 k  word lines among the 2 m  word lines is defective. Consequently, redundant word lines are effectively employed, write and verify operations are stable, and thereby the yield and performance of the semiconductor memory device are improved.

This application is a divisional application filed under 37 CFR §1.53(b)of parent application Ser. No. 10/150,017, filed May 20, 2002, which inturn is a divisional of application Ser. No. 09/832,916, filed Apr. 12,2001, now U.S. Pat. No. 6,414,874, which in turn is a divisional ofapplication Ser. No. 09/457,736, filed Dec. 10, 1999, now U.S. Pat. No.6,288,945, which in turn is a divisional of application Ser. No.09/081,243, filed May 19, 1998, now abandoned which in turn is adivisional of application Ser. No. 08/822,036, filed Mar. 24, 1997, nowU.S. Pat. No. 5,815,440, which in turn is a divisional of applicationSer. No. 08/432,723, filed Jun. 6, 1995, now U.S. Pat. No. 5,666,314,which in turn is a divisional of application Ser. No. 08/079,738, filedJun. 22, 1993, now U.S. Pat. No. 5,452,251.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device, and moreparticularly, to a flash memory, i.e., a nonvolatile semiconductormemory device having electrically and collectively erasablecharacteristics.

2. Description of the Related Art

Recently, in accordance with popularization of computers, wordprocessors, and the like, a plurality of semiconductor memory devices,typically non-volatile semiconductor memory devices, e.g., flash memory,used in such information processors, have been developed and produced.

The flash memory, which is one kind of non-volatile semiconductor memorydevice, can be made programmable by the user. Further, the flash memorycan be rewritten by electrically and collectively erasing the storeddata, and then by programming. Therefore, the flash memories haveattracted considerable attention as a replacement for magnetic storagedevices because they are suitable for integration. Note, there is anecessity of improving redundant circuits, write circuits, andovererasing preventive measures for such a flash memory.

SUMMARY OF THE INVENTION

An object of a first aspect of the present invention is to provide asemiconductor memory device for realizing effective word line redundancyand stable write and verify operations in a semiconductor memory device,to improve the yield and performance of the semiconductor memory device.

An object of a second aspect of the present invention is to provide asemiconductor memory device for reducing the size of the semiconductormemory device, to improve the yield of large capacity semiconductormemories and reduce the cost thereof.

An object of a third aspect of the present invention is to provide asemiconductor memory device for carrying out a delivery test of asemiconductor memory device with “n” rewrite operations at the maximum,and taking into account deterioration due to an increase in the numberof rewrite operations, to guarantee the maximum rewrite operations N(N>n) for a user.

An object of a fourth aspect of the present invention is to provide asemiconductor memory device that is capable of supplying a write drainvoltage that is not influenced by the threshold voltage of a writevoltage supply transistor, thereby correctly writing data to a memorycell even with a low write voltage.

An object of a fifth aspect of the present invention is to provide asemiconductor memory device that correctly reads data even if there isan overerased cell transistor.

An object of a sixth aspect of the present invention is to provide asemiconductor memory device that correctly reads data by savingovererased cell transistors.

An object of a seventh aspect of the present invention is to provide asemiconductor memory device capable of simultaneously erasing aplurality of blocks of memory cells and easily verifying the erasedblocks.

An object of an eighth aspect of the present invention is to provide asemiconductor memory device employing two power sources that are easy touse and operable like a single power source.

An object of a ninth aspect of the semiconductior memory device of thepresent invention is to provide an inexpensive decoder circuit thatsolves the problems of the prior art. The decoder circuit of the presentinvention is simple and compact to achieve the full selection andnonselection of word or bit lines in a test mode. This decoder circuitis suitable for high integration.

According to a first aspect of the present invention, there is provideda semiconductor memory device comprising 2^(n) word lines, a pluralityof bit lines, a plurality of nonvolatile memory cells each formed of aMIS transistor disposed at each intersection of the word lines and thebit lines, and a threshold voltage of the MIS transistor beingexternally electrically controllable, a write circuit for writing datato a memory cell located at an intersection of selected ones of the wordlines and the bit lines, a sense amplifier for reading data out of thememory cells, a first unit for simultaneously selecting a block of 2^(m)(n>m) word lines among the 2^(n) word lines, and a second unit for notselecting a block of 2^(k) (m>k) word lines among the 2^(m) word lines,the second unit not selecting the block of 2^(k) word lines andselecting a block of 2^(k) word lines prepared outside the 2^(n) wordlines when any one of the 2^(k) word lines among the 2^(m) word lines isdefective.

The selected word lines may receive a negative voltage, and theunselected word lines receive a zero or positive voltage. The block of2^(n) word lines may form a real cell block, the block of 2^(m) wordlines may form an erase block, and the block of 2^(k) word lines outsidethe block of 2^(n) word lines may form a redundant cell block.

Further, according to a first aspect of the present invention, there isprovided a semiconductor memory device comprising 2^(n) word lines, aplurality of bit lines, a plurality of nonvolatile memory cells eachformed of a MIS transistor disposed at each intersection of the wordlines and the bit lines, and a threshold voltage of the MIS transistorbeing externally electrically controllable, a write circuit for writingdata to a memory cell located at an intersection of selected ones of theword lines and the bit lines, a sense amplifier for reading data out ofthe memory cells, a first unit for simultaneously selecting a block of2^(m) (n>m) word lines among the 2^(n) word lines, and a second unit fornot selecting a block of 2^(k) (m>k) word lines among the 2^(m) wordlines, data being written to any memory cell transistor, which iscontained in the 2^(k) word lines and whose threshold voltage is lowerthan the potential of an unselected word line, so that the thresholdvoltage of the memory cell transistor exceeds the potential of theunselected word line, and a block of 2^(k) word lines prepared outsidethe 2^(n) word lines being used as redundant word lines.

Further, according to a first aspect of the present invention, there isalso provided a semiconductor memory device comprising a plurality ofword lines, a plurality of bit lines, a plurality of nonvolatile memorycells each formed of a MIS transistor disposed at each intersection ofthe word lines and the bit lines, and a threshold voltage of the MIStransistor being externally electrically controllable, a write circuitfor writing data to a memory cell located at an intersection of selectedones of the word lines and the bit lines, and a sense amplifier forreading data out of the memory cells, wherein each word line iscontrolled such that a drain current of a memory cell transistorconnected to the word line is lower than a channel current thereof, whenwriting data to the cell transistor to increase the threshold voltage ofthe memory cell transistor to be higher than the potential of anunselected word line.

Each word line may be controlled by applying a signal in accordance witha pulse signal.

Furthermore, according to a first aspect of the present invention, thereis also provided a semiconductor memory device comprising a plurality ofword lines, a plurality of bit lines, a plurality of nonvolatile memorycells each formed of a MIS transistor disposed at each intersection ofthe word lines and the bit lines, and a threshold voltage of the MIStransistor being externally electrically controllable, a write circuitfor writing data to a memory cell located at an intersection of selectedones of the word lines and the bit lines, and a sense amplifier forreading data out of the memory cells, wherein an output current of thesense amplifier is changed according to a combination of ON states oftwo load transistors having different capacities, to realize a normaldata read operation, an erase verify operation, and a write verifyoperation.

A reference voltage may be increased to provide a word line with avoltage, which is used to carry out the write verify or erase verifyoperations on any cell transistor connected to the word line. P-channeltype and n-channel type transistors fabricated in the same process maybe connected in series like diodes to provide a word line with a voltagewhich is used to carry out the write verify or erase verify operationson any cell transistor connected to the word line.

The semiconductor memory device may be constituted by a flash memory.

According to a second aspect of the present invention, there is provideda semiconductor memory device comprising a plurality of real memorycells divided into blocks, a plurality of redundant memory cells to bereplaced with defective ones of the real memory cells, a plurality ofdefective address specifying units for specifying defective addresses ofthe respective blocks of the real memory cells, and an address comparingunit shared by the defective address specifying unit, for comparing thedefective addresses with addresses in the blocks of the real memorycells.

Further, according to a second aspect of the present invention, there isalso provided a semiconductor memory device comprising a real cell arrayhaving a plurality of memory cells, and a plurality of redundant cellsto be replaced with defective memory cells of the real cell array, aredundant information storing cell array for writing data to a defectiveaddress according to an externally provided address, a cell selectioncircuit for selecting the redundant information storing cell arrayaccording to the externally provided address, and a read circuit forreading an output of the redundant information storing cell arrayselected by the cell selection circuit, and providing a redundancysignal.

According to a third aspect of the present invention, there is provideda semiconductor memory device comprising electrically erasablenonvolatile memory cells to and from which data is automatically writtenand erased according to an internal algorithm incorporated in thesemiconductor memory device, wherein the allowable value of write orerase operations is carried out according to the internal algorithmbeing variable.

According to a fourth aspect of the present invention, there is provideda semiconductor memory device comprising a plurality of word lines, aplurality of bit lines, a plurality of memory cells each formed of a MIStransistor disposed at each intersection of the word lines and the bitlines, and a threshold voltage of the MIS transistor being externallyelectrically controllable, and a write voltage supply transistor forsupplying a write voltage to a drain of the memory cell, wherein thewrite voltage supply transistor is formed of a p-channel type MIStransistor, which effectively applies the write voltage to the drain ofthe memory cell.

Further, according to a fourth aspect of the present invention, there isalso provided a semiconductor memory device comprising a plurality ofword lines, a plurality of bit lines, a plurality of memory cells eachformed of a MIS transistor disposed at each intersection of the wordlines and the bit lines, and a threshold voltage of the MIS transistorbeing externally electrically controllable, and a write voltage supplytransistor for supplying a write voltage to a drain of the memory cell,wherein the write voltage supply transistor is formed of an n-channeltype MIS transistor, and the semiconductor memory device comprises astep-up unit being disposed to increase a gate voltage of the writevoltage supply transistor at least up to a sum of the write voltage anda threshold voltage of the write voltage supply transistor.

According to a fifth aspect of the present invention, there is provideda semiconductor memory device comprising a plurality of word lines, aplurality of bit lines, a memory cell array including a plurality ofmemory cells each formed of a MIS transistor disposed at eachintersection of the word lines and the bit lines, a threshold voltage ofthe MIS transistor being externally electrically controllable accordingto charges to be injected to a floating gate thereof, and the floatinggates of the MIS transistors being simultaneously discharged tocollectively erase the memory cells, a first power source for applying anormal selection voltage to a selected word line to select memory cellsconnected to the word line, when reading data, and a second power sourcefor establishing an unselected state on unselected word lines includingmemory cells that have been overerased by the collective erasing, whenreading data.

Further, according to a fifth aspect of the present invention, there isalso provided a semiconductor memory device comprising a plurality ofword lines, a plurality of bit lines, a memory cell array including aplurality of memory cells each formed of a MIS transistor disposed ateach intersection of the word lines and the bit lines, a thresholdvoltage of the MIS transistor being externally electrically controllableaccording to charges to be injected to a floating gate thereof, and thefloating gates of the MIS transistors being simultaneously discharged tocollectively erase the memory cells, a first row decoder for applying anormal voltage to a selected word line to select memory cells connectedto the word line, when reading data, and a second row decoder forapplying a predetermined source voltage to the source of each memorycell connected to the selected word line, and applying an unselectedstate establishing voltage to the sources of memory cells, includingthose overerased by the collective erasing, connected to unselected wordlines, when reading data.

According to a sixth aspect of the present invention, there is provideda semiconductor memory device comprising a plurality of word lines, aplurality of bit lines, a memory cell array including a plurality ofmemory cells each formed of a MIS transistor disposed at eachintersection of the word lines and the bit lines, a threshold voltage ofthe MIS transistor being externally electrically controllable accordingto charges to be injected to a floating gate thereof, and the floatinggates of the MIS transistors being simultaneously discharged tocollectively erase the memory cells, wherein a method of savingovererased memory cells of the semiconductor memory device detectsmemory cells that have been overerased by the collective erasing, andwriting data to the overerased memory cells, thereby saving theovererased memory cells.

Further, according to a sixth aspect of the present invention, there isalso provided a semiconductor memory device comprising a plurality ofword lines, a plurality of bit lines, a memory cell array including aplurality of memory cells each formed of a MIS transistor disposed ateach intersection of the word lines and the bit lines, and a thresholdvoltage of the MIS transistor being externally electrically controllableaccording to charges to be injected to a floating gate thereof, awrite-before-erase unit for writing all memory cells of the memory cellarray before erasing them, an erase unit for erasing all of the writtenmemory cells by the write-before-erase unit and for verifying theerasing, an overerased cell detecting unit for detecting overerasedmemory cells among the erased and verified memory cells by the eraseunit, and an overerased cell saving unit for writing the overerasedmemory cells detected by the overerased cell detecting unit, therebysaving the overerased memory cells.

According to a seventh aspect of the present invention, there isprovided a semiconductor memory device comprising a plurality of wordlines, a plurality of bit lines, and a plurality of nonvolatile memorycells each formed of a MIS transistor disposed at each intersection ofthe word lines and the bit lines, and a threshold voltage of the MIStransistor being externally electrically controllable, wherein thenonvolatile memory cells are divided into a plurality of cell blocks tobe selected according to a block selection signal provided by a blockaddress buffer, each of the cell blocks has a data erasing unit and alatching unit for latching the block selection signal, and thereby dataof the cell blocks that have latched the block selection signal aresimultaneously erased.

According to an eighth aspect of the present invention, there isprovided a semiconductor memory device comprising a first terminal forreceiving a normal voltage, a second terminal for receiving a highvoltage from a high-voltage supply unit, and the high voltage beingrequired to write or erase data and higher than the normal voltagerequired to read data, a third terminal for providing the high-voltagesupply unit with a control signal that controls the supply of the highvoltage.

Further, according to an eighth aspect of the present invention, thereis also provided a semiconductor memory device comprising a step-upcircuit for supplying a high voltage that is required to write or erasedata and higher than a normal voltage required to read data, a commanddetermination unit that determines whether or not an operation specifiedby an input command to the semiconductor memory device requires the highvoltage, and provides a control signal to start the supply of the highvoltage if the operation requires the high voltage, and if not, acontrol signal to stop the high voltage.

Furthermore, according to an eighth aspect of the present invention,there is also provided a computer system having a semiconductor memorydevice as a part of a storage unit and a step-up circuit for generatinga high voltage required to write and erase data to and from thesemiconductor memory device, wherein the computer system comprises acontrol unit for automatically generating a control signal to controlthe step-up circuit, in response to an access operation to thesemiconductor memory device.

According to a ninth aspect of the present invention, there is provideda semiconductor memory device comprising a plurality of word lines, aplurality of bit lines, a plurality of memory cells disposed at eachintersection of the word lines and the bit lines, and a decoder circuitfor selecting the memory cell according to an address signal in a normaldecoding function and for carrying out a full selection operation or anonselection operation of the word lines or the bit lines in a testfunction, and an output row or a decoding row connected to a first powersource and a second power source, the first power source supplying ahigh voltage, and the second power source supplying a reference voltageor the high voltage in response to a control signal.

Further, according to a ninth aspect of the present invention, there isalso provided a semiconductor memory device comprising a plurality ofword lines, a plurality of bit lines, a plurality of memory cellsdisposed at each intersection of the word lines and the bit lines, adecoder circuit for selecting the memory cell according to an addresssignal in a normal decoding function and for carrying out a fullselection operation or a nonselection operation of the word lines or thebit lines in a test function, and an output row or a decoding rowconnected to a first power source and a second power source, the firstpower source supplying a reference voltage, and the second power sourcesupplying a reference voltage or the high voltage in response to acontrol signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from thedescription of the preferred embodiments as set forth below withreference to the accompanying drawings, wherein:

FIG. 1 is diagram for explaining the operation of a memory cell used fora semiconductor memory device related to the first aspect of the presentinvention;

FIG. 2 is a block diagram showing a conventional semiconductor memorydevice related to the first aspect of the present invention;

FIG. 3 is a circuit diagram showing a column address buffer of thesemiconductor memory device of FIG. 2;

FIG. 4 is a circuit diagram showing a row address buffer of thesemiconductor memory device of FIG. 2;

FIG. 5 is a circuit diagram showing a row decoder of the semiconductormemory device of FIG. 2;

FIG. 6 is a circuit diagram showing a column decoder of thesemiconductor memory device of FIG. 2;

FIG. 7 is a circuit diagram showing a write circuit of the semiconductormemory device of FIG. 2;

FIG. 8 is a circuit diagram showing a source voltage supply circuit ofthe semiconductor memory device of FIG. 2;

FIG. 9 is a circuit diagram showing a sense amplifier of thesemiconductor memory device of FIG. 2;

FIG. 10 is a diagram showing an example of write characteristics curvesof the semiconductor memory device of FIG. 2;

FIG. 11 is a circuit block diagram showing an embodiment of asemiconductor memory device according to the first aspect of the presentinvention;

FIG. 12 is a circuit diagram showing a row address buffer of thesemiconductor memory device of FIG. 11;

FIG. 13 is a circuit diagram showing a row decoder of the semiconductormemory device of FIG. 11;

FIG. 14 is a circuit diagram showing a coincidence circuit of thesemiconductor memory device of FIG. 11;

FIG. 15 is a circuit diagram showing an essential part of a row decoderof the semiconductor memory device of FIG. 11;

FIG. 16 is a diagram showing a waveform of a signal applied to thecircuit of FIG. 15;

FIG. 17 is a circuit diagram showing a verify voltage generator of thesemiconductor memory device of FIG. 11;

FIG. 18 is a circuit diagram showing a sense amplifier of thesemiconductor memory device of FIG. 11;

FIG. 19 is a circuit diagram showing a logic circuit for producing acontrol signal to the sense amplifier of FIG. 18;

FIG. 20 is a circuit block diagram showing a redundant circuit of aconventional semiconductor memory device related to the second aspect ofthe present invention;

FIG. 21 is a circuit block diagram showing an arrangement of theconventional redundant circuits of FIG. 20;

FIG. 22 is a block diagram showing a semiconductor memory deviceemploying the conventional redundant circuit of FIG. 20;

FIG. 23 is a circuit block diagram showing an embodiment of a redundantcircuit for the semiconductor memory device according to the secondaspect of the present invention;

FIG. 24 is a diagram showing real cells and redundant cells of asemiconductor memory device employing the redundant circuit of FIG. 23;

FIG. 25 is a block diagram showing a semiconductor memory deviceemploying the redundant circuit of FIG. 23;

FIG. 26 is a circuit block diagram showing another embodiment of aredundant circuit for the semiconductor memory device according to thesecond aspect of the present invention;

FIG. 27 is a circuit block diagram showing still another embodiment of aredundant circuit for the semiconductor memory device according to thesecond aspect of the present invention;

FIG. 28 is a block diagram showing a semiconductor memory deviceemploying the redundant circuit of FIG. 27;

FIG. 29 is a flowchart showing an example of an internal write algorithmfor a semiconductor memory device according to the third aspect of thepresent invention;

FIG. 30 is a block diagram showing an embodiment of a semiconductormemory device according to the third aspect of the present invention;

FIG. 31 is a circuit diagram showing an essential part of thesemiconductor memory device of FIG. 30;

FIG. 32 is a timing chart for explaining the operation of the circuit ofFIG. 31;

FIG. 33 is diagram for explaining the operation of a memory cell of asemiconductor memory device according to the fourth aspect of thepresent invention;

FIG. 34 is a circuit block diagram showing an example of a conventionalsemiconductor memory device related to the fourth aspect of the presentinvention;

FIG. 35 is a circuit block diagram showing an embodiment of asemiconductor memory device according to the fourth aspect of thepresent invention;

FIG. 36 is a circuit diagram showing an essential part of anotherembodiment of a semiconductor memory device according to the fourthaspect of the present invention;

FIG. 37 is a circuit block diagram showing a conventional semiconductormemory device related to the fifth aspect of the present invention;

FIG. 38 is a circuit diagram showing a row decoder of the semiconductormemory device of FIG. 37;

FIG. 39 is a circuit diagram showing a column decoder of thesemiconductor memory device of FIG. 37;

FIG. 40 is a circuit diagram showing bit line transfer gates of thecolumn decoder of FIG. 39;

FIG. 41 is a circuit block diagram showing an embodiment of asemiconductor memory device according to the fifth aspect of the presentinvention;

FIG. 42 is a circuit diagram showing a row decoder of the semiconductormemory device of FIG. 41;

FIG. 43 is a circuit block diagram showing another embodiment of asemiconductor memory device according to the fifth aspect of the presentinvention;

FIG. 44 is a circuit block diagram showing first and second row decodersof the semiconductor memory device of FIG. 43;

FIG. 45 is a circuit diagram showing a part of the second row decoder ofFIG. 44;

FIG. 46 is a circuit block diagram showing an essential part of anembodiment of a semiconductor memory device according to the sixthaspect of the present invention;

FIG. 47 is a circuit diagram showing a sense amplifier of thesemiconductor memory device of FIG. 46;

FIG. 48 is a block diagram schematically showing a system employing thesemiconductor memory device according to the sixth aspect of the presentinvention;

FIG. 49 is a flowchart for explaining an example of a process carriedout by the semiconductor memory device of the sixth aspect of thepresent invention;

FIG. 50 is diagram for explaining the operation of a memory cell of asemiconductor memory device according to the seventh aspect of thepresent invention;

FIG. 51 is a circuit diagram showing a conventional semiconductor memorydevice related to a semiconductor memory device according to the seventhaspect of the present invention;

FIG. 52, consisting of FIGS. 52A and 52B, is a circuit block diagramshowing an embodiment of a semiconductor memory device according to theseventh aspect of the present invention;

FIG. 53 is a circuit diagram showing a source voltage supply circuit ofthe semiconductor memory device of FIG. 52;

FIG. 54 is a circuit diagram showing an expected value storage circuitof the semiconductor memory device of FIG. 52;

FIG. 55 is a circuit diagram showing a coincidence circuit of thesemiconductor memory device of FIG. 52;

FIG. 56, consisting of FIGS. 56A and 56B, is a circuit block diagramshowing another embodiment of a semiconductor memory device according tothe seventh aspect of the present invention;

FIG. 57 is a circuit diagram showing an expected value generator of thesemiconductor memory device of FIG. 56;

FIG. 58, consisting of FIGS. 58A and 58B, is a circuit block diagramshowing still another embodiment of a semiconductor memory deviceaccording to the seventh aspect of the present invention;

FIG. 59 is a circuit diagram showing a block selection signal storagecircuit of the semiconductor memory device of FIG. 58;

FIG. 60 is a circuit diagram showing a write circuit of thesemiconductor memory device of FIG. 58;

FIG. 61 is a circuit diagram showing a data inversion circuit of thesemiconductor memory device of FIG. 58;

FIG. 62 is a block diagram showing a conventional system using asemiconductor memory device related to the eighth aspect of the presentinvention;

FIG. 63 is a flowchart showing an example of the control operation forcontrolling the system of FIG. 62;

FIG. 64 is a diagram for explaining the principle of a semiconductormemory device according to the eighth aspect of the present invention;

FIG. 65 is a flowchart showing the steps of processing a high voltagerequiring operation in a system employing a semiconductor memory deviceaccording to the eighth aspect of the present invention;

FIG. 66 is a block diagram showing a total system using a semiconductormemory device according to the eighth aspect of the present invention;

FIG. 67 is a block diagram showing an embodiment of a semiconductormemory device according to the eighth aspect of the present invention;

FIG. 68 is a flowchart showing an example of the control operation forcontrolling the system of FIG. 66;

FIG. 69 is a circuit block diagram showing a voltage test circuit shownin FIG. 67;

FIG. 70 is a circuit block diagram for explaining the start of a nextoperation a predetermined time after the transmission of a controlsignal;

FIG. 71 is a diagram schematically showing an example of a DC-DCconverter;

FIG. 72 is a diagram schematically showing another example of a DC-DCconverter;

FIG. 73 is a block diagram showing a semiconductor memory device havingDC-DC converter according to the eighth aspect of the present invention;

FIG. 74 is a circuit diagram showing a conventional decoder circuit usedin a semiconductor memory device related to the ninth aspect of thepresent invention;

FIG. 75 is a circuit diagram showing another conventional decodercircuit used in a semiconductor memory device related to the ninthaspect of the present invention;

FIG. 76 is a circuit diagram showing still another conventional decodercircuit used in a semiconductor memory device related to the ninthaspect of the present invention;

FIG. 77 is a circuit diagram showing still another conventional decodercircuit used in a semiconductor memory device related to the ninthaspect of the present invention;

FIG. 78 is a circuit block diagram showing an embodiment of a decodercircuit used in a semiconductor memory device according to the ninthaspect of the present invention;

FIG. 79 is a circuit block diagram showing another embodiment of adecoder circuit used in a semiconductor memory device according to theninth aspect of the present invention;

FIG. 80A is a circuit block diagram showing still another embodiment ofa decoder circuit used in a semiconductor memory device according to theninth aspect of the present invention;

FIG. 80B is a modification of the decoder circuit shown in FIG. 80A;

FIG. 81A is a circuit block diagram showing still another embodiment ofa decoder circuit used in a semiconductor memory device according to theninth aspect of the present invention;

FIG. 81B is a modification of the decoder circuit shown in FIG. 81A;

FIG. 82 is a circuit block diagram showing an example of a semiconductormemory device using a decoder circuit according to the ninth aspect ofthe present invention;

FIGS. 83A and 83B are circuit diagrams showing the details of thedecoder circuit of FIG. 78;

FIGS. 84A and 84B are circuit diagrams showing the details of thedecoder circuit of FIG. 79;

FIGS. 85A and 85B are circuit diagrams showing the details of thedecoder circuit of FIG. 80A;

FIGS. 86A and 86B are circuit diagrams showing the details of thedecoder circuit of FIG. 81A;

FIG. 87 is a circuit diagram showing a power supply circuit for thedecoder used in a semiconductor memory device according to the ninthaspect of the present invention;

FIG. 88 is a circuit diagram showing a conventional test mode signaldetector of a semiconductor device related to the ninth aspect of thepresent invention;

FIG. 89 is a timing chart for explaining the problems of the detector ofFIG. 88;

FIG. 90 is a circuit diagram showing a test mode signal detector of asemiconductor device according to the ninth aspect of the presentinvention;

FIG. 91 is a timing chart of the detector of FIG. 90; and

FIG. 92 shows a decoder circuit shown in FIG. 85B, using the detectors930 shown in FIG. 90.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For a better understanding of the preferred embodiments, the problems ofthe related art will be explained, with reference to FIGS. 1 to 10.

FIG. 1 shows a memory cell (MC) of an electrically collectively erasablenonvolatile semiconductor memory device (a flash memory) for which afirst aspect of the present invention is applied. The memory cell (celltransistor) MC has a floating gate FG. The floating gate FG is locatedbetween a source and a drain and is insulated from other regions. Acontrol gate CG is formed above the floating gate FG.

To write data to the cell transistor MC, a drain voltage Vd applied to adrain DD is substantially equalized with a power source voltage Vcc. Agate voltage Vg applied to the control gate CG is a positive highvoltage (about +10 volts at the maximum). A source voltage Vs applied toa source SS is zero. Electrons are injected from the drain DD to thefloating gate FG, to write data “0” to the cell transistor MC. The drainvoltage Vd may be a write voltage Vpp, if the voltage Vpp is available.The high gate voltage Vg may be the write voltage Vpp, or may beproduced by increasing the power source voltage Vcc.

To erase data from the cell transistor MC, the gate voltage Vg is set toa high negative value (about −10 volts at the lowest). The drain voltageVd is opened to put the drain DD in a floating state. The source voltageVs is equalized with the power source voltage Vcc. Electrons are pulledfrom the floating gate FG to the source SS, to erase data from the celltransistor MC, i.e., to write data “1” to the cell transistor. To readdata from the cell transistor MC, the gate voltage Vg is equalized tothe power source voltage Vcc, the drain voltage Vd to about one volt,and the source voltage Vs to zero. Then, it is checked to see whether ornot a drain current flows, to determine if the data stored in the celltransistor MC is “1” or “0”.

FIG. 2 shows a conventional semiconductor memory device related to thefirst aspect of the present invention. This semiconductor memory devicehas a row address buffer 111, a row decoder 112, a column address buffer113, a column decoder 114, a data I/O buffer 115, a write circuit 116, asense amplifier 117, a negative voltage generator 118, and a sourcevoltage supply circuit 119. The semiconductor memory device involves bitlines BLs, word lines WLs, a write control signal W that will be highlevel H when writing data, and an erase control signal E that will behigh level H when erasing data.

When reading data from a memory cell (cell transistor) MC, a row addressis given to select a word line WL, and a column address to select a bitline BL, thereby selecting the memory cell. The sense amplifier 117senses whether or not a current flows through the selected memory cell,to determine whether data stored in the cell is “1” or “0”.

When writing data to a memory cell MC, the write control signal W is setto high level H. The write circuit 116 provides a bus BUS with a writevoltage, and the column decoder 114 connects the bus BUS to a given bitline BL. The row decoder 112 applies the write voltage to a selectedword line WL. When erasing data from memory cells MCs, the erase controlsignal E is set to high level H. The source voltage supply circuit 119applies an erase voltage to source lines of the memory cells MCs, andthe column address buffer 113 puts the bit lines BLs in an unselectedstate. The row address buffer 111 simultaneously selects a given numberof word lines WLs, and the row decoder 112 sets the selected word linesWLs to low level L and the unselected word lines WLs to high level H.The negative voltage generator 118 applies a negative voltage to theword lines WLs of low level L.

FIGS. 3 to 6 show examples of the column address buffer 113, row addressbuffer 111, row decoder 112, column decoder 114 of the semiconductormemory device of FIG. 2, respectively.

When reading data from a memory cell MC, the erase control signal E isat low level L. The column address buffer 113 of FIG. 3 and row addressbuffer 111 of FIG. 4 provide positive and negative logic values withrespect to an input address. The row decoder 112 of FIG. 5 involves asignal φ that oscillates at a given frequency when erasing or writingdata, and a signal φ_(R) that is at high level H for some period of timeafter the reception of the input address.

When reading data, the write control signal W is at low level L.Transistors T₁ and T₂ of the row decoder 112 of FIG. 5 introduce thepower source voltage Vcc. According to an address input (an output ofthe row address buffer 111), a predetermined decoder is selected. Forexample, a node N₃ of FIG. 5 becomes high level H. If the signal φ_(R)provides a pulse of high level H in this state, nodes N₂ and N₄ arereset to zero. When the signal φ_(R) restores low level L, the node N₂is charged to the power source voltage Vcc. Due to the self-bootstrapeffect of transistors T₆ and T₇, the node N₄ is also charged to thelevel of the power source voltage Vcc. The column decoder 114 operatessimilarly to the row decoder 112. As a result, the power source voltageVcc is applied to a predetermined word line WL, and a predetermined bitline BL is connected to the sense amplifier 117.

FIGS. 7 and 8 show examples of the write circuit 116 and source voltagesupply circuit 119 of the semiconductor memory device of FIG. 2,respectively.

With the write control signal W of high level H, data of low level L,and an inverted data signal /DATA of high level H, the write circuit 116of FIG. 7 increases the power source voltage Vcc to provide the bus BUSwith a high voltage. With this high voltage, data is written to a givencell transistor. The signal /DATA is transferred as a write signal fromthe data I/O buffer 115 to the write circuit 116. Note, a mark “/”denotes an inverted level or inverted signal, and thus, for example, thesignal /DATA denotes an inverted level of signal DATA.

When erasing data, the erase control signal E is raised to high level H.In the column address buffer 113 of FIG. 3, both outputs A and /A areset to low level L. These outputs A and /A are applied to the columndecoder 114 to put the columns (bit lines BLs) in an unselected state.Namely, the bit lines BLs are electrically disconnected from every node.In the row address buffer 111, the erase control signal E is applied to“m” buffer elements among “n” buffer elements in total. As a result, therow decoder 112 of FIG. 5 simultaneously selects 2^(m) word lines. Inthe row decoder 112, the erase control signal E is at high level H, sothat the node N₂ receives zero volts and the node N₆ receives high levelH. As a result, the selected word lines WLs are set to low level L, andthe unselected word lines WLs are set to high level H.

The negative voltage generator 118 provides an erase voltage to the wordlines WLs of low level L. The word lines WLs at high level H aremaintained at high level H because the potential of an output N₆ of aNOR gate of FIG. 5 is always at low level L, and therefore, the signal φis not transmitted to a capacitance element connected to the node N₆. Atthis time, the source voltage supply circuit 119 of FIG. 8 applies thepower source voltage Vcc to the source SS of each cell transistor MC. Asa result, the cell transistors of the 2^(m) word lines aresimultaneously erased.

FIG. 9 shows an example of the sense amplifier 117 of the semiconductormemory device of FIG. 2.

An output of the sense amplifier 117 of FIG. 9 will be high level H orlow level L depending on whether or not the drain current of a selectedcell transistor MC is larger than the allowable current of a transistorT₈. Transistors T₉, T₁₀, T₁₁, and T₁₂, form a bias circuit that sets thepotential of the bus BUS to about one volt.

When writing data, the write control signal W is set to high level H,and the signal φ is oscillated at a predetermined frequency. Thetransistors T₄ and T₅ provide the node N₁ with a write voltage. Similarto the read operation with the signal φ_(R), a pulse of high level H isapplied to charge the node N₂ to the write voltage. The node N₄ is alsocharged to the same level as the node N₂ due to the self-bootstrapeffect of the transistors T₆ and T₇. The column decoder 114 operates inthe same manner. Consequently, the write voltage is supplied to aselected word line WL, and a transistors with spare cell transistors.Namely, the normal operation will never resume by replacing word linescontaining the overerased cells with redundant word lines. In this case,the overerased cells may be rewritten to cancel the overerased state andrestore the normal operation. The overerased cells, however, pass alarger current around a point A of FIG. 10 to make such writingdifficult.

Below, the preferred embodiments of a semiconductor memory deviceaccording to the present invention will be explained, with reference tothe accompanying drawings.

First, a semiconductor memory device according to a first aspect of thepresent invention will be explained with reference to FIGS. 11 to 19.

FIG. 11 shows the semiconductor memory device (flash memory) accordingto the first aspect of the present invention. This memory differs fromthe conventional semiconductor memory device of FIG. 2 in that itadditionally has a coincidence circuit 120 for comparing an inputaddress with a defective address, and a redundant row decoder 130. A rowaddress buffer 101, a row decoder 102, a column address buffer 103, acolumn decoder 104, a data I/O buffer 105, a write circuit 106, a senseamplifier 107, a negative voltage generator 108, and a source voltagesupply circuit 109 of this embodiment correspond to the row addressbuffer 111, row decoder 112, column address buffer 113, column decoder114, data I/O buffer 115, write circuit 116, sense amplifier 117,negative voltage generator 118, and source voltage supply circuit 119 ofthe related art of FIG. 2; the row address buffer 101, the row decoder102, and the coincidence circuit 120 act as a first means forsimultaneously selecting a block of word lines. Row address buffer 101,row decoder 102, coincidence circuit 120, and redundant row decoder 130act as a second means for deselecting a first block of word line, aswill be discussed below.

The operation of the embodiment will now be explained. When writing orreading data to or from a memory cell (cell transistor) of thesemiconductor memory device, the coincidence circuit 120 compares aninput address with each defective address stored therein. If they agreewith each other, the coincidence circuit 120 provides the row addressbuffer 101 and redundant row decoder 130 with an output signal to putthe row decoder 102 in an unselected state and the redundant row decoder130 in a selected state. This enables a redundant cell to be accessedinstead of a defective cell. When erasing data from the semiconductormemory device, an erase control signal E of high level H is provided tothe column address buffer 103, row address buffer 101, row decoder 102,coincidence circuit 120, and redundant row decoder 130.

If there is no defective cell (transistor) in a memory cell array, i.e.,if the redundancy is not used, a normal erase operation is carried outas previously explained. A redundancy control signal RED from thecoincidence circuit 120 indicates no redundancy.

If there is a defective cell, the coincidence circuit 120 stores theaddress of the defective cell. In this embodiment, there are 2^(n) wordlines in total, and 2^(m) word lines among the 2^(n) word lines form anerase block. The embodiment employs 2^(k) redundant (spare) word lines.To write or read data, the number of bits of defective address storageelements in the coincidence circuit 120 must be “n−k.” Since an eraseblock includes 2^(m) word lines, the number of address bits necessaryfor selecting the erase block is “n−m.” When erasing data, “n−m”addresses are provided to select a block of 2^(m) word lines. The inputaddresses are compared with upper “n−m” address bits among the “n−k”address bits stored in the coincidence circuit 120. If they agree witheach other, it is determined that the erase block of 2^(k) word linesincludes a defective cell.

Address information representing the block of 2^(k) word lines includingthe defective cell among the block of 2^(m) word lines is given by theremaining “m−k” address bits stored in the coincidence circuit 120.Consequently, the row decoder 102 sets the block of 2^(k) word linesspecified with the “m−k” address bits among the block of 2^(m) wordlines to be unselected. When the coincidence of “n−m” address bits isfound in erasing 2^(m) word lines, the redundant row decoder 130 isactivated to erase the 2^(k) redundant word lines.

FIGS. 12 to 14 show essential parts of the row address buffer 101, rowdecoder 102, and coincidence circuit 120 of the semiconductor memorydevice of FIG. 11.

In FIG. 12, there are “n” buffers in the row address buffer 101. Lower“m” buffers receive the erase control signal E to select 2^(m) wordlines in an erase operation. Any one of the m buffers receives theredundancy control signal RED. If this signal is high level H in a writeor read operation, it means that an input address agrees with adefective address, and therefore, the word line WL is set to beunselected.

The redundancy control signal RED is an output signal of the coincidencecircuit 120 of FIG. 14. The coincidence circuit 120 includes addressstorage fuses used to specify a block of 2^(k) word lines among the2^(n) word lines, and a fuse RUSE used to store the state of use ofredundancy. Since the erase control signal E is at low level L except inan erase operation, the redundancy control signal RED becomes high levelH only when all pieces of fuse information agree with input informationpieces. In the erase operation, the redundancy control signal REDbecomes high level H if upper “n−m” addresses A_(RBm+1) to A_(RBn)coincide with input addresses. Fuse data of “m−k” addresses A_(RBk+1) toA_(RBm) are provided directly to a NAND gate of FIG. 13. As a result, aclock of 2^(k) word lines among a block of 2^(m) word lines is set to beunselected. The redundancy control signal RED is also provided to theredundant row decoder 130. In this way, an optional block of 2^(k) wordlines in an erase block of 2^(m) word lines is replaceable with a blockof redundant (spare) word lines.

If a memory cell is overerased in the semiconductor memory device (flashmemory), it deteriorates the yield. The bit lines of the semiconductormemory device are shared by the redundant cells and real cells, andtherefore, it is impossible to save the overerased cell by replacing itwith the redundant cell. For example, if a memory cell (cell transistor)MC₁₁ is overerased in FIG. 11, the cell MC₁₁ may be replaced with aredundant cell MCR₁₁. In this case, however, the overerased cell MC₁₁passes current even when a word line WL1 is at low level L. If thishappens, data “0” of a cell existing on the bit line BL1 will not becorrectly read. This problem is easily solved by writing data “0” in theovererased cell MC₁₁ before replacing it with the redundant cell MCR₁₁.Namely, electrons are injected into the floating gate of the cell MC₁₁before replacing it with the redundant cell.

The overerasing of a memory cell will be explained with reference toFIG. 10, which shows the write characteristics curve of a semiconductormemory device. The floating gate of an overerased cell is positivelycharged to increase a current at a point A on the curve, thus disablingdata write. To solve this problem, it is necessary to control the gatelevel of any cell when writing data to the cell, such that a currentflowing through the cell does not exceed the load curve of the writecircuit 106 around the point A. This is easily achieved by continuouslyproviding pulses to a corresponding word line WL during the data write.When the word line WL receives the pulses, it surely realizes a curve Cindicated with a dotted line in FIG. 10 when moving from low level L tohigh level H, or from high level H to low level L. Consequently, thedata write is enabled irrespective of the condition of the floatinggate.

FIG. 15 shows an essential part of an example of the row decoder 102 ofthe semiconductor memory device of FIG. 11, and corresponds to the inputportion B of the row decoder L12 of the conventional semiconductormemory device of FIG. 5. A NOR gate of FIG. 15 receives a signal φWwhose waveform is shown in FIG. 16. With this signal, the potential ofthe node N₂ of FIG. 5 is continuously oscillated between zero and awrite potential, to continuously provide pulses to a word line WL.Instead of applying the pulses, it is possible to employ a circuit forapplying an intermediate voltage to a word line. Generally, a write orerase operation of a semiconductor memory device is followed by a verifyoperation, which is achieved by applying a verify voltage to read datafrom a word line. The verify voltage must be constant irrespective ofchanges in environmental conditions of the semiconductor memory devicesuch as a change in a power source voltage. Accordingly, it is practicalto provide the verify voltage by increasing a reference potential (Vss)of the semiconductor memory device.

FIG. 17 shows an example of a verify voltage generator 150 of thesemiconductor memory device of FIG. 11. This circuit generates a verifyvoltage applied to the node N₁ of the row decoder circuit 112 (102) ofFIG. 5.

The verify voltage generator 150 of FIG. 17 includes a clamp circuit151, an oscillator 152, and a step-up circuit 153. The clamp circuit 151has transistors T₁₃ and T₁₄ for determining a clamp voltage. Thesetransistors are p-channel type and n-channel type MOS (MIS) transistorsconnected in series as diodes. According to a CMOS process, the channelregions of transistors are simultaneously fabricated to cancelfluctuations in the thresholds of the transistors. This results instabilizing the clamp voltage.

An n-channel type MOS transistor T₁₅ has a threshold of about zerovolts. This transistor provides the oscillator 152 with the clampvoltage. The step-up circuit 153 operates according to a low powersource voltage (ground voltage) Vss. A verify voltage at the node N₁,therefore, is stable regardless of the power source voltage. Erase andwrite verify operations are carried out with different voltages.Different clamp voltages are easily produced by changing the number oftransistors (T₁₃, T₁₄, . . . ) of the clamp circuit 151. A signal VRbecomes high level H when carrying out the verify operation. The verifyoperation may be carried out by changing a decision current of the senseamplifier.

FIG. 18 shows an example of the sense amplifier 107 of the flash memoryof FIG. 11. The sense amplifier 107 has p-channel type transistors T_(L)_(¹) and T_(L) _(²) serving as load transistors. The current supplycapacities of these transistors are T_(L) _(¹) >T_(L) _(²) . The flashmemory has three modes, i.e., an erase verify mode, a normal read mode,and a write verify mode. The total capacity of the load transistors forthese modes must be the erase verify mode>the normal read mode>the writeverify mode. The circuit of FIG. 18 realizes this relationship withV_(R) _(¹) =V_(R) _(²) =L for the erase verify mode, V_(R) _(¹) =L andV_(R) _(²) =H for the normal read mode, and V_(R) _(¹) =H and V_(R) _(²)=L for the write verify mode.

FIG. 19 shows an example of a logic circuit for providing the controlsignals V_(R) _(¹) and V_(R) _(²) to the sense amplifier of FIG. 18. Thelogic circuit of FIG. 19 involves a write verify signal WV and an eraseverify signal EV. This circuit is advantageous in simplifying the powersource of the row decoder when producing the verify voltage. The loadcontrol logic circuit is applicable for the erase verify operation ofthe flash memory.

A semiconductor memory device according to a second aspect of thepresent invention will be explained with reference to FIGS. 20 to 28.

FIG. 20 shows a redundant circuit 210 of a conventional semiconductormemory device related to the second aspect of the present invention. Theredundant circuit 210 has a fuse 211 serving as defective addressspecifying means for storing a defective address. The fuse 211 isconnected or disconnected to provide high (H) or low (L) level. Acomparator 214 compares the information of the fuse 211 with an externalinput address, and if they agree with each other, provides an addresscoincidence signal of, for example, high level H.

FIG. 21 shows a redundant arrangement 2100, which contains a pluralityof the conventional redundant circuits of FIG. 20. Outputs of theredundant circuits 210 are passed through a NAND gate and an inverter,to provide a redundancy signal. Only when each input address agrees withthe information of the fuse 211 in every redundant circuit 210, is aredundancy signal of high level H provided to read data from redundantcells.

FIG. 22 shows a semiconductor memory device employing the conventionalredundant arrangement 2100 of FIG. 21. When the redundant arrangement2100 provides a redundancy signal, a real cell selector 217 prohibitsreading of a real cell 218, and a redundant cell selector 215 reads aredundant cell 216, thereby replacing a defective real cell with aredundant cell. A data read circuit 219 reads data out of a selected oneof the redundant cell 216 and real cell 218.

According to the prior art of FIGS. 20 to 22, each fuse (211) requiresone address comparator (214). When many defective memory cells must bereplaced with redundant cells, a corresponding numbers of fuses andaddress comparators must be prepared. This results in increase in thesize and cost of a semiconductor chip.

In this way, the conventional redundant technique has a problem ofincreasing the size and cost of a semiconductor memory device when thenumbers of memory cells and redundant cells increase.

An object of a second aspect of the present invention is to provide asemiconductor memory device for reducing the size of the semiconductormemory device, to improve the yield of large capacity semiconductormemories and reduce the cost thereof.

FIG. 23 shows a redundant circuit of a semiconductor memory deviceaccording to the second aspect of the present invention. This redundantcircuit 200 employs a transistor T_(A), a fuse 201A, a transistor T_(B),and a fuse 201B in place of the fuse 211 of the conventional redundantcircuit 210 of FIG. 20. The fuses 201A and 201B are selected accordingto the logic of an external input address An (/An, an inversion of theaddress An). The address An (/An) is an upper address for selecting oneof the blocks of real cells. The fuses 201A and 201B of the redundantcircuit 200 share an address comparator 204, to reduce the total numberof the address comparators 204 and the size and cost of thesemiconductor memory device.

FIG. 24 shows an arrangement of real cells 208 and redundant cells 206in the semiconductor memory device employing the redundant circuit ofFIG. 23. The real cells 208 are divided into a first real cell block208A to be selected with a block selection address An of low level L,and a second real cell block 208B to be selected with a block selectionaddress An of high level H and an inverted address /An of low level L.The redundant cells 206 are divided into a first redundant cell block206A to be replaced with some cells of the first real cell block 208A,and a second redundant cell block 206B to be replaced with some cells ofthe second real cell block 208B. When an address (An-1, An-2, . . . ,etc) that is common for the real cell blocks 206A and 206B includes adefective cell, a predetermined number of real cells including thedefective cell among the block specified by the block address An arereplaced with the redundant cells.

FIG. 25 shows a semiconductor memory device employing the redundantcircuit of FIG. 23 according to the present invention. When theredundant circuit 200 provides a redundancy signal, a real cell selector207 prohibits the reading of a real cell 208, and a redundant cellselector 205 reads a redundant cell 206. Unlike the semiconductor memorydevice of FIG. 22, the semiconductor memory device of FIG. 25 providesan address (a block selection address An) input to the redundant cellselector 205 as well, which selects one of the redundant cells 206A and206B corresponding to the fuses 201A and 201B of the redundant circuit200 in response to a logic of the address An. Namely, the redundant cellselector 205 receives the block address An and selects one of theredundant cells 206A and 206B corresponding to one of the real cellblocks 208A and 208B that is selected according to the block address An.In FIG. 25, a data read circuit 209 reads data out of a selected one ofthe redundant cell 206 and real cell 208.

As explained above, the address comparator 204 is shared by theredundant cell blocks 206A and 206B when replacing a defective real cellwith a redundant cell. In the above explanation, the block selectionaddress An is of one bit for selecting one of the two real cell blocks.The block selection address may involve two bits An and An-1 to selectone of the four real cell blocks and one of the four redundant cellblocks.

FIG. 26 shows another redundant circuit 200′ for a semiconductor memorydevice according to the second aspect of the present invention. Theredundant circuit 200′ includes a cell selector 220, redundantinformation storing cell arrays 221 and 223, and read circuits 222 and224.

The redundant circuit 200′ has the two redundant information storingcell arrays 221 and 223 and the two read circuits 222 and 224.

Each of the redundant information storing cell arrays 221 and 223includes nonvolatile memory cell transistors such as EPROMs and is usedto write data to defective addresses according to external inputaddresses. The cell selector 220 selects one of the cell arrays 221 and223 according to an address input. The read circuits 222 and 224 providefour redundancy signals through AND gates 225A, 225B, 225C, and 225D,and through inverters 226A and 226B. This embodiment provides two-bitdata from the two cell arrays 221 and 223 in parallel, so that fourdefective cells can be replaced with redundant cells. It is alsopossible to provide data of three bits or more in parallel.

FIG. 27 shows still another redundant circuit 200″ according to thesecond aspect of the present invention, and FIG. 28 shows asemiconductor memory device employing the redundant circuit of FIG. 27.

Unlike the redundant circuit 200′ of FIG. 26 that provides a pluralityof bits (two bits) in parallel, the embodiment of FIG. 27 provides asingle bit to replace a plurality of defective real cells 208 withredundant cells 206.

The redundant circuit 200″ reads a single bit according to an addressinput, and according to a logic of the bit, provides a redundancysignal. In the semiconductor memory device of FIG. 28, the redundantcircuit 200″ provides a redundancy signal to a redundant cell selector205′ and to a real cell selector 207′. A part (a real cell blockselecting address An) of an address input is also supplied to theselectors 205′ and 207′, to determine redundant cells to be replacedwith defective real cells.

A semiconductor memory device according to a third aspect of the presentinvention will be explained with reference to FIGS. 29 to 32.

Some electrically erasable programmable nonvolatile semiconductormemories such as flash memories have an internal algorithm forautomatically writing or erasing data.

The writing or erasing of data to a flash memory is carried out byapplying a write pulse and by executing a read operation (a verifyoperation). If the read operation determines that the writing isinsufficient, the write pulse is again applied to repeat the writing orerasing. The maximum number of write pulses applied, i.e., the number ofverify operations is prescribed in specifications. These write, erase,and verify operations are externally controlled.

Some flash memories incorporate an algorithm for automatically carryingout the write or erase operation. A user of this kind of flash memoriesis usually provided with the allowable number of write or eraseoperations.

Delivery tests of the flash memories sometimes require repetitiverewrite operations. This results in deteriorating the number of write orerase operations guaranteed for a user. In this case, the flash memoriesthat have passed the delivery test may fail on the user side beforereaching the guaranteed number of write or erase operations.

An object of a third aspect of the present invention is to provide asemiconductor memory device for carrying out a delivery test of asemiconductor memory device with “n” rewrite operations at the maximum,and taking into account deterioration due to an increase in the numberof rewrite operations, to guarantee the maximum rewrite operations N(N>n) for a user.

FIG. 29 is a flowchart showing an algorithm to be stored in asemiconductor memory device according to the third aspect of the presentinvention.

After the start of a write process, step S301 applies a write pulse.Step S302 carries out a verify operation. Namely, step S302 readswritten data and determines whether or not the data has beensufficiently written. If the result is sufficient, the write processends. If it is insufficient, step S303 checks to see whether or not thenumber of applied write pulses has reached a predetermined number N. Ifit is NO, steps S301 and S302 are repeated. If it is YES, the writeprocess aborts. Namely, it is determined that the data has not beencorrectly written to a cell transistor after the application of N writepulses.

In this way, the third aspect of the present invention carries out adelivery test of a semiconductor memory device with “n” write pulses atthe maximum, which is smaller than an allowable number “N”. Namely, thedelivery test is carried out under severer conditions than normalconditions, thereby guaranteeing the allowable number N for a user.

FIG. 30 shows a semiconductor memory device according to the thirdaspect of the present invention. This memory includes a write controller311, a write pulse generator 312, a cell array 313, a pulse counter 314,a switch 315, a stop signal generator 316, and a high voltage detector317.

The write controller 311 receives an external control signal and a writestop signal WS, to control the write pulse generator 312 to write datainto cell transistors of the cell array 313. The write pulse generator312 provides a write pulse to the cell array 313 and to the pulsecounter 314, which counts the number of write pulses, i.e., the numberof verify operations. To easily change a count in the pulse counter 314,direct input wiring to a NAND circuit and output wiring of an invertermay be fabricated in advance, to selectively connect the NAND circuit tothe wiring during manufacturing.

The switch 315 is connected to the standard number “N” of pulseapplications or to the delivery test maximum “n” (n<N) of pulseapplications. According to the number N or n, the stop signal generator316 provides the write controller 311 with a write stop signal WS. Theswitching of the switch 315 is controlled in response to a switchcontrol signal SC provided by the high voltage detector 317 depending onwhether or not an external high voltage is applied thereto.

FIG. 31 shows an essential part of the semiconductor memory device ofFIG. 30, and FIG. 32 explains the operation of the circuit of FIG. 31.In FIGS. 31 and 32, a reference mark QCOi is a pulse count signal ofeach stage.

A stop signal WS(N) corresponding to the standard maximum of pulseapplications is prepared according to the pulse count signals QCO2,QCO3, and QCO4. A stop signal WS(n) corresponding to the delivery testmaximum of pulse applications is prepared according to the pulse countsignals QCO0, QCO1, and QCO2. When switch control signal SC from thehigh voltage detector (EWCMGN) 317 is at low level L, i.e., when no highvoltage is applied, the stop signal WS(N) corresponding to the standardnumber N is provided with the 21st pulse of the pulse signal QCO0. Whenthe signal SC from the detector 317 is at high level H, i.e., when ahigh voltage is applied to a given terminal, the stop signal WS(n)corresponding to the delivery test maximum n is provided with the 4thpulse of the pulse signal QCO0.

In the above explanation, the maximum number of write pulses applieduntil the stop signal WS is provided is changed between the standardnumber guaranteed for a user and the number for a delivery test. Insteadof changing the pulse application numbers, the width of a write pulsemay be changed. For example, the width of a write pulse for a deliverytest may be shorter than that of a write pulse for standard use, toimpose severer conditions on the delivery test. The width of a writepulse for the delivery test may be narrowed once a high voltage isapplied to a given terminal. Although the above explanation is relatedto a write operation, the same is applicable for an erase operation.

In this way, a semiconductor memory device according to the third aspectof the present invention incorporates an internal algorithm that iscapable of changing the number of write or erase operations of memorycells. Semiconductor memory devices with this arrangement will rarelyfail on the user side once passing a delivery test.

A semiconductor memory device according to a fourth aspect of thepresent invention will be explained with reference to FIGS. 33 to 36.

FIG. 33 shows a memory cell (MCo) of the semiconductor memory device(flash memory) according to the fourth aspect of the present invention.The memory cell (cell transistor) has a floating gate FG. The floatinggate is disposed between a source and a drain and is insulated fromother regions. A control gate CG is formed above the floating gate FG.

To read data from the cell transistor, a gate voltage Vg is equalizedwith a power source voltage Vcc, a drain voltage Vd to about one volt,and a source voltage Vs to a ground level Vss. The data is determined tobe “1” or “0” depending on whether or not a drain current flows.

To write data to the cell transistor, the drain voltage Vd applied to adrain DD is set to a high voltage (usually Vcc<high voltage<Vpp), thegate voltage Vg applied to the control gate CG to the write voltage Vpp(up to about +10 volts), and the source voltage Vs applied to a sourceSS to the ground level Vss. Charges are injected from the drain DD tothe floating gate FG, to write data of “0.” The write voltage isbecoming lower in these days. Accordingly, it is required to effectivelyapply such low write voltage to the drain.

FIG. 34 shows a conventional flash memory related to the fourth aspectof the present invention. The flash memory includes a row address buffer411, a row decoder 412, a column address buffer 413, a column decoder414, a buffer 415, a write voltage supply transistor 416, a senseamplifier 417, and a bus 418. The flash memory also includes bit linesBLs, word lines WLs, write data /WD (inverted level), and a writecontrol signal W.

To read data from a memory cell MCo, a word line WL and a bit line xLare selected according to a row address and a column address,respectively. The sense amplifier 417 checks to see whether or not acurrent flows through the selected memory cell (cell transistor) MCo, todetermine whether data stored in the memory cell is “1” or “0.”

To write data to a memory cell MCo, the write control signal W sets wordand bit line selection signals to a write voltage Vpp. When write data/WD is provided, the transistor 416 is turned ON so that the writevoltage Vpp (decreased by the threshold voltage of the transistor 416)is applied to the bus 418 and to the drain of the memory cell. The flashmemory of FIG. 34 employs a sufficiently high voltage as the writevoltage Vpp, so that the write voltage supply transistor 416 may be ann-channel type MOS transistor. In this case, the write voltage Vpp isdecreased by the threshold voltage of the transistor and is applied tothe drain of the cell transistor MCo.

Recent flash memories employ a single power source of, for example, fivevolts, which requires a reduced write voltage. With such a five-voltsingle power source, the threshold voltage of the write voltage supplytransistor 416 may decrease a supply voltage lower than the writevoltage Vpp, to supply an insufficient write voltage to the drain of amemory cell.

An object of the fourth aspect of the present invention is to provide asemiconductor memory device that is capable of supplying a write drainvoltage with no influence of the threshold voltage of a write voltagesupply transistor, thereby correctly writing data to a memory cell evenwith a low write voltage.

FIG. 35 shows a semiconductor memory device according to an embodimentof the fourth aspect of the present invention. Unlike the conventionalsemiconductor memory device of FIG. 34, the semiconductor memory deviceof FIG. 35 employs a p-channel type MOS transistor 406 as a writevoltage supply transistor, and a buffer 405 receives a positive logicsignal WD as write data. A row address buffer 401, a row decoder 402, acolumn address buffer 403, a column decoder 404, and the buffer 405 ofFIG. 35 correspond to the row address buffer 411, row decoder 412,column address buffer 413, column decoder 414, and buffer 415 of therelated art of FIG. 34, respectively.

The semiconductor memory device of FIG. 35 involves bit lines BLs, wordlines WLs, and a write control signal W.

To read data from a memory cell MCo of the semiconductor memory deviceof FIG. 35, a word line WL and a bit line BL are selected according to arow address and a column address, respectively. A sense amplifier 407checks to see whether or not a current flows through the selected memorycell (cell transistor) MCo, to determine whether the memory cell storesdata “1” or “0.”

To write data to a memory cell, the write control signal W sets word andbit line selection signals to a write voltage Vpp. The buffer 405converts write data WD into a signal having the write voltage Vpp. Whenthe gate signal to the write voltage supply transistor 406 becomes lowlevel L, the transistor 406 is turned ON to supply the write voltage Vppto a bus 408. Since the transistor 406 is a p-channel type MOStransistor, the write voltage Vpp applied to the source of thetransistor 406 is not dropped by the threshold voltage of the transistor406. As a result, the potential of the bus 408 is increased to nearlythe write voltage Vpp, thereby effectively applying the write voltagevpp to the drain of the selected cell transistor MCo. Even when thisflash memory is driven with a single five-volt power source, the lowwrite voltage Vpp is sufficient to write data to memory cells.

FIG. 36 is a circuit diagram showing an essential part of asemiconductor memory device according to another embodiment of thefourth aspect of the present invention. The part shown in FIG. 36corresponds to the voltage supply transistor 416 and buffer 415 of theconventional semiconductor memory device of FIG. 34.

The embodiment of FIG. 36 employs an n-channel type MOS transistor as awrite voltage supply transistor 426, similar to the prior art of FIG.34. Instead of the buffer 415 of FIG. 34, however, the embodiment ofFIG. 36 employs n-channel type MOS transistors 4251, 4252, and 4253,inverters 4255, 4256, and 4257, and a bootstrap circuit 4253 made of acapacitor. When write data WD changes from high level H to low level L,the bootstrap circuit 4253 sets the gate of the transistor 4251 to highlevel H. Then, the gate of the transistor 4252 changes to low level L,to increase the potential of a node N40. At this time, the capacitor4253 further increases the level of the gate of the transistor 4251 upto about Vpp+Vcc. This potential is applied to the gate of the writevoltage supply transistor 426, to increase the potential of a bus 418 toabout the write voltage Vpp.

In this way, the gate of the write voltage supply transistor 426 made ofan n-channel type MOS transistor receives the data signal increased tothe write voltage Vpp or over, and the bus 418 receives a voltage equalto the write voltage Vpp, so that data is effectively written to aselected memory cell with the low write voltage Vpp.

As explained above, the semiconductor memory device according to thefourth aspect of the present invention provides a sufficient write drainvoltage even under the influence of the threshold voltage of a writevoltage supply transistor, to correctly write data to a selected memorycell even with a low write voltage.

A semiconductor memory device according to a fifth aspect of the presentinvention will be explained with reference to FIGS. 37 to 45.

FIG. 37 shows a conventional semiconductor memory device (flash memory)related to the fifth aspect of the present invention. The semiconductormemory device of FIG. 37 includes a row decoder 512, a column decoder514, a sense amplifier 517, and a source voltage supply circuit 519. Thesemiconductor memory device includes memory cells (cell transistor) MCseach made of an n-channel type MOS (MIS) transistor, word lines WLs, bitlines BLs, and source lines SLs. The source voltage supply circuit 519is connected to the sources of the memory cells MCs in a memory cellarray through the source lines SLs, to electrically collectively erasethe memory cells. Each memory cell MC is the same as that shown in FIG.33.

FIG. 38 shows an example of the row decoder 512, FIG. 39 shows anexample of the column decoder 514, and FIG. 40 shows a bit line transfergate 5145 of the column decoder 514 of FIG. 39.

In FIG. 38, the row decoder 512 has a power source 5121, gates RG1 toRGn for receiving a row address, a transistor 5122 connected between thegates RG1 to RGn and the power source 5121, and an inverter (transistors5123 and 5124) connected between the power source 5121 and a low voltagesupply source Vss (at a ground level GND of zero volts). The invertercontrols the level of a word line WL. For example, a selected word lineWL corresponding to a row address with all high level (H) bits to turnON the gates RG1 to RGn receives the output Vcc of the power source 5121through the transistor 5123, and every other unselected word line WLreceives the low voltage Vss of zero volts through the transistor 5124.

In FIG. 39, the column decoder 514 has a power source 5141, gates CGI toCGm for receiving a column address, a transistor 5142 between the gatesCG1 to CGm and the power source 5141, a bit line transfer gate 5145, andan inverter (transistors 5143 and 5144) between the power source 5121and the low voltage Vss. The inverter controls the bit line transfergate 5145. When input column address bits are each of high level H toturn ON the gates CG1 to CGm, a selected bit line BL is connected to thesense amplifier 517.

In FIG. 40, bit line transfer gates 51451 to 5145m are connected to thesense amplifier 517 through a bus BUS. Only a bit line selected throughthe bit line transfer gates 51451 to 5145m is connected to the senseamplifier 517. Accordingly, the contents of a memory cell MC located atan intersection of the selected word and bit lines are provided outsidethrough the sense amplifier 517.

Generally, all cell transistors (memory cells) MCs of a flash memory aresimultaneously erased for the sake of simplicity of circuit design. Theerasing is repeated until all the cell transistors are completelyerased. Due to statistical reasons, some cell transistors in a flashmemory are relatively easily erased and some are not. If acharacteristic difference between these two kinds of memory cells isvery large, the memory cells that are easily erased may be overerased.The characteristics of cell transistors easily deviate from one anotherduring a wafer processing and through the repetition of write and eraseoperations.

To write or erase data to or from cell transistors of a flash memory,the floating gates of the cell transistors are charged or discharged. Ifa memory cell is overerased, the floating gate thereof holds chargeswhose polarity is opposite to a polarity formed when data is writtenthereto. Namely, when a memory cell has an oppositely charged floatinggate, it is determined that the cell has been overerased.

Nonvolatile semiconductor memories (flash memories) usually have aNOR-type cell array. According to this type, drains of cell transistors(n-channel type MOS transistors) are connected to one another bit lineby bit line. When selecting a cell transistor (memory cell), thecommonly connected cell transistors are uniformly biased, and then, apositive bias voltage is applied only to the gate of the cell transistorto be selected and zero volts is applied to the gates of unselected celltransistors.

The sources of all cell transistors are grounded through the sourcevoltage supply circuit 519 in FIG. 37. Since the cell transistors areenhancement MOS (MIS) transistors (n-channel type MOS transistors), theunselected cell transistors do not pass current, and only the selectedcell transistor passes or does not pass a current depending on thequantity of charges accumulated in the floating gate thereof. Accordingto the current flowing through the selected cell transistor, data “0” or“1” is determined.

When an enhancement cell transistor of the flash memory is overerased,the characteristics of the overerased transistor change to those of adepletion transistor. To solve this problem, the NOR-type flash memorysupplies no current to unselected cell transistors when no bias isapplied to the gates of the transistors. The overerased cell transistor,however, shows depletion characteristics, and therefore, passes acurrent even if it is unselected. Namely, even if a selected celltransistor passes no current, the overerased unselected cell transistormay pass a current, to erroneously determine data “0” as data “1,” ordata “1” as data “0.”

An object of the fifth aspect of the present invention is to provide asemiconductor memory device that correctly reads data even if there isan overerased cell transistor.

FIG. 41 shows a flash memory according to an embodiment of the fifthaspect of the present invention. The flash memory has a row decoder 502,a column decoder 504, a sense amplifier 507, and a source voltage supplycircuit 509. The arrangement of FIG. 41 is basically the same as that ofFIG. 37 and is characterized by the row decoder 502.

FIG. 42 shows an example of the row decoder 502 of FIG. 41. The rowdecoder 502 has a positive power source 5021 for generating a givenpositive voltage, gates RG1 to RGn for receiving a row address, atransistor 5022 between the gates RG1 to RGn and the positive powersource 5021, a negative power source 5025, and an inverter (transistors5023 and 5024) between the positive power source 5021 and the negativepower source 5025. The inverter controls the level of a word line WL.When an input row address has bits each of high level H, the gates RG1to RGn are turned ON, and a corresponding word line (selected word line)WL receives an output Vcc of the positive power source 5021 through thetransistor 5123. At this time, other unselected word lines WLs receiveeach an output (a negative voltage) of the negative power source 5025through the transistor 5024. The negative power source 5025 may be thenegative voltage generator 118 of the semiconductor memory device of therelated art of FIG. 5.

When the output of the negative power source 5025 is applied to the gateof an overerased cell transistor MC showing depletion characteristics,the voltage causes the transistor to be unselected to pass no current.Namely, the negative output voltage of the negative power source 5025suppresses the gate voltage of the overerased depletion n-channel typeMOS transistor below a threshold voltage. Consequently, even if aselected bit line involves overerased cell transistors, the contents ofa cell transistor selected by a word line are correctly read out throughthe sense amplifier 507.

FIG. 43 shows a semiconductor memory device according to anotherembodiment of the fifth aspect of the present invention. Thissemiconductor memory device has first and second row decoders 5221 and5222 corresponding to the row decoder 502 and source voltage supplycircuit 509 of FIG. 41. The second row decoder 5222 applies a voltageequal to or greater than the level of a selected bit line to sourcelines SWLs corresponding to unselected word lines WLs. A column decoder524, a sense amplifier 527, etc., of FIG. 43 are the same as those ofFIG. 41.

When reading data from a memory cell MC, the first row decoder 5221applies a normal voltage Vcc to a selected word line WL, to selectmemory cells (cell transistors) connected to the selected word line WL.The second row decoder 5222 applies a low power source voltage Vss ofzero volts to the source line SWL of the cell transistors connected tothe selected word line, and a voltage equal to or greater than the drainvoltage of the selected bit line to the source lines of cell transistorsconnected to unselected word lines. Overerased cell transistors amongthe unselected transistors can be disconnected because the gate voltagethereof becomes lower than the source voltage thereof. A voltage appliedto the sources of the memory cells connected to the unselected wordlines in the read operation may be equal to the level of the selectedbit line. Even if a channel is produced due to the overerasing, nocurrent flows if there is no potential difference between the drain andsource of each overerased cell. Accordingly, the overerased unselectedcell transistors never influence the read operation.

FIG. 44 shows examples of the first and second row decoders 5221 and5222 of FIG. 43, and FIG. 45 shows a part of the second row decoder5222.

In FIG. 44, the first row decoder 5221 has a power source (Vcc) 52211, aNAND gate 52212 for receiving a row address, and an inverter 52213. Thesecond row decoder 5222 has a power source (Vcc) 52221, a NAND gate52222 for receiving the row address, inverters 52223 and 52224, and apower source 52225. The power source 52225 supplies a voltage higherthan the drain voltage of a selected bit line to the sources of celltransistors connected to unselected word lines when reading data, asshown in FIG. 45.

As explained above, a flash memory according to the fifth aspect of thepresent invention correctly reads data of “0” or “1” even if some memorycells have been overerased during a wafer process or through repetitivewrite and read operations. The fifth aspect of the present inventionthus improves the yield and reliability of flash memories.

A semiconductor memory device according to a sixth aspect of the presentinvention will be explained with reference to FIGS. 46 to 49. Similar tothe fifth aspect, the sixth aspect correctly reads data from memorycells of a flash memory even if some memory cells are overerased duringa collective erase operation.

As explained before with reference to FIGS. 37 to 40, cell transistors(memory cells) of a flash memory are simultaneously erased for the sakeof simplicity of circuit designing. The erasing is repeated until allthe cell transistors are completely erased. Some cell transistors of theflash memory are relatively easily erased and some are not. When thecells are collectively erased, the cells that are easily erased may beovererased. The characteristics of the cell transistors easily differfrom one another during a wafer process and through the repetition ofwrite and erase operations. Accordingly, the overerasing of cellsfrequently occurs.

Generally, flash memories have a NOR-type cell array. According to thistype, drains of n-channel type MOS transistors (cell transistors) areconnected to one another bit line by bit line. When selecting a celltransistor, the commonly connected cell transistors are uniformlybiased, and then, a positive bias voltage is applied to only the gate ofthe cell transistor to be selected and zero volts is applied to thegates of unselected cell transistors. Since the cell transistors areenhancement n-channel type MOS transistors, the unselected celltransistors do not pass a current, and only the selected cell transistorpasses or does not pass a current depending on the quantity of chargesstored in the floating gate thereof. According to the current flowingthrough the selected cell transistor, data “0” or “1” is determined.

When an enhancement cell transistor of the flash memory is overerased,the characteristics of the overerased transistor change to those of adepletion transistor. To solve this problem, the NOR-type flash memorysupplies no current to unselected cell transistors when no bias isapplied to the gates of the transistors. The overerased cell transistor,however, shows depletion characteristics, and therefore, passes currenteven if it is unselected. Namely, even if a selected cell transistorpasses no current, the overerased unselected cell transistor passescurrent, to erroneously determine data “0” as data “1” or data “1” asdata “0.”

An object of the sixth aspect of the present invention is to provide asemiconductor memory device that correctly reads data by savingovererased cell transistors.

FIG. 46 shows an essential part of the semiconductor memory deviceaccording to the sixth aspect of the present invention. Thesemiconductor memory device includes a row decoder 602, a column decoder604, a sense amplifier 607, memory cells (cell transistors) MCs eachmade of an n-channel type MOS transistor, word lines WLs, and bit linesBLs. Each memory cell MC is the same as that shown in FIG. 33. Note, thesense amplifier 607 receives a signal OECK which is changed to highlevel H when an erase verifying operation is carried out (with referenceto Step 613 in FIG. 49).

FIG. 47 shows an example of the sense amplifier 607 of FIG. 46. Thesense amplifier 607 includes n-channel type MOS transistors 6071, 6072,6073, 6074, 6075, and 6077, and p-channel type MOS transistors 6076 and6078. Note, as shown in FIG. 47, the signal OECK, which is applied tothe sense amplifier 607, is also applied to the gate of the MOStransistors 6077.

To detect overerased cell transistors after a collective eraseoperation, the row decoder 602 sets all word lines WL1, WL2, . . . ofthe collectively erased cell array to low level L. Thereafter, thecolumn decoder 604 successively selects column gates G601, G602, . . . ,to successively connect the bit lines BL1, BL2, . . . to the senseamplifier 607. Whenever a bit line involving an overerased celltransistor is connected to the sense amplifier 607, the sense amplifier607 provides an output of low level L. Then, the transistor 6077 of thesense amplifier 607 is turned ON to increase a drive current of thesense amplifier 607, thereby preventing a current from flowing throughthe overerased cell. At the same time, the row decoder 602 scans theword lines WL1, WL2, . . . , to detect the overerased cell that hascaused the sense amplifier 607 to provide an output of high level H.

FIG. 48 schematically shows a system employing the semiconductor memorydevice according to the sixth aspect of the present invention. Thesystem includes the flash memory 610, a read only memory (ROM) 620, anda central processing unit (CPU) 630.

The ROM 620 stores an algorithm shown in FIG. 49, and the CPU 630controls the flash memory 610 according to the algorithm. Namely, theCPU 630 saves overerased cells of the flash memory 610 according to thealgorithm stored in the ROM 620.

FIG. 49 shows a flow of the algorithm stored in the ROM 620 of FIG. 48.After the start of an erase operation of the flash memory, step S611carries out a write-before-erase operation. Namely, before collectivelyerasing the memory cell array of the flash memory, data “0” is writtento every cell transistor of the memory cell array.

Step S612 carries out the collective erase operation, and step S613carries out an erase verify operation. Note, in the step S613, a signalOECK, which is applied to the sense amplifier 607 and the gate of theMOS transistor 6077, is changed from low level L to high level H, whenthe erase verify operation is carried out. Namely, electrons aregradually discharged from the floating gates of all cell transistors ofthe memory cell array, to achieve the erase operation. Step S614 checksto see whether or not there is an overerased cell. If there is noovererased cell, the erase process ends. If there is an overerased cell,step s615 is carried out. As explained with reference to FIGS. 46 and47, step S614 detects overerased cell transistors one by one.

Step S615 writes data to the overerased cell (overerased bit) detectedin step S614. Step S616 carries out the same test as step S614 to checkfor overerasing. If the overerased cell detected in step S614 restores anormal erased state due to the write operation of step S615, and ifthere is no other overerased cell, step S617 carries out the same eraseverify operation as step S613. If the erase verify operation of stepS617 is successful, the erase process ends. If the erase verifyoperation of step S617 fails, steps S618 and S619 carry out the eraseand erase verify operations.

If the overerased cell detected in step S614 restores a normal erasedstate due to the write operation of step S615 and if another overerasedcell is found in step S616, step S615 writes data to the overerased cellnewly found. In this way, data is written to every overerased cell ofthe memory cell array, to restore a normal erased state in everyovererased cell.

The erase process of FIG. 49 is stored in the ROM 620 of the system ofFIG. 48, and the CPU 630 reads and executes the erase process.Alternatively, the erase process of FIG. 49 may be realized by hardware.For example, the flash memory itself may incorporate a logic circuitthat achieves the erase process.

As explained above, the sixth aspect of the present invention savesovererased cell transistors and correctly reads data. Even if memorycells of a semiconductor memory device are overerased during a waferprocess or through the repetition of write and erase operations, thesixth aspect of the present invention correctly reads data out of thesemiconductor memory device, thereby improving the yield and reliabilityof the semiconductor memory device.

A semiconductor memory device according to a seventh aspect of thepresent invention will be explained with reference to FIGS. 50 to 61.

FIG. 50 shows a memory cell (MC) of an electrically collectivelyerasable nonvolatile semiconductor memory device (a flash memory)according to the seventh aspect of the present invention. The memorycell (cell transistor) has a floating gate FG. The floating gate isdisposed between a source and a drain and is insulated from otherregions. A control gate CG is formed above the floating gate FG.

To write data to the cell transistor, a drain voltage Vd applied to adrain DD is set to, for example, 6 V, a gate voltage Vg applied to thecontrol gate CG to a write voltage (erase voltage) Vpp, and a sourcevoltage Vs applied to a source SS to zero. Charges are injected from thedrain DD to the floating gate FG, to write data of “0.”

To erase data from the cell transistor, the gate voltage Vg and drainvoltage Vd are set to a floating state, and the source voltage Vs is setto the erase voltage Vpp. Electrons are extracted from the floating gateFG to the source SS, to erase the cell transistor, i.e., to write data“1” to the cell transistor. To read data from the cell transistor, thegate voltage Vg is set to a power source voltage Vcc, the drain voltageVd to about one volt, and the source voltage Vs to zero. It is tested tosee whether or not a drain current flows, to determine whether thewritten data is “1” or “0.”

FIG. 51 shows a conventional semiconductor memory device related to theseventh aspect of the present invention. The semiconductor memory deviceincludes a block address buffer 710, block selecting gates 7101 and7102, a row address buffer 711, a row decoder 712, a column addressbuffer 713, a column decoder 714, a data I/O buffer 715, a write circuit716, a sense amplifier 717, and source voltage supply circuits 7191 and7192. The semiconductor memory device also includes bit lines BLs, wordlines WLs, memory cells MCs, a write control signal W that becomes highlevel H when writing data, and an erase signal E that becomes high levelH when erasing data.

This semiconductor memory device operates similarly to the semiconductormemory device of FIG. 2. The semiconductor memory device of FIG. 51additionally has the block address buffer 710 and block selecting gates7101 and 7102 corresponding to blocks B1 and B2 of memory cells. One ofthe block selecting gates 7101 and 7102 is selected according to a blockselection signal from the block address buffer 710, to connect one ofthe blocks B1 and B2 to the write circuit 716 or the sense amplifier717. In each of the blocks B1 and B2 of memory cells, sources of thememory cells are connected to one another. With the use of the sourcevoltage supply circuits 7191 and 7192 provided for the blocks B1 and B2,respectively, the blocks are separately erasable.

To erase the cells of one of the blocks B1 and B2, the erase signal E ofhigh level H is supplied to the row address buffer 711 and columnaddress buffer 713, to set the outputs of the buffers 711 and 713 to anunselected logic (for example, each complementary output being at lowlevel L). This results in putting all word and bit lines WLs and BLs inan unselected state. The erase signal E and a block selection signalfrom the block address buffer 710 are supplied to the source voltagesupply circuits 7191 and 7192. One of the source voltage supply circuitsselected with the block selection signal of, for example, high level H,is used to supply an erase voltage Vpp to the selected block.Consequently, all cells in the selected block are erased.

When writing data, the write control signal W of high level H isprovided to the row address buffer 711 and column address buffer 713.Accordingly, a selected word line WL is set to the write level Vpp, anda selected bit line BL is connected to the write circuit 716 through oneof the block selection gates 7101 and 7102 selected according to theblock selection signal. The write circuit 716 provides the selected bitline BL of the selected block with a write voltage of, for example, 6volts to write data.

The conventional semiconductor memory device of FIG. 51 may erase ablock of memory cells but is unable to simultaneously erase a pluralityof blocks of memory cells. Namely, the semiconductor memory device ofFIG. 51 sequentially erases blocks one after another, and thereafter,verifies the erased blocks. Accordingly, this semiconductor memorydevice requires a long time for erasing a plurality of blocks and mustcarry out a complicated verify process.

An object of the seventh aspect of the present invention is to provide asemiconductor memory device capable of simultaneously erasing aplurality of blocks of memory cells and easily verifying the erasedblocks.

FIG. 52 shows a semiconductor memory device according to an embodimentof the seventh aspect of the present invention. This semiconductormemory device includes a block address buffer 701, expected valuestorage circuits 7021 and 7022, coincidence circuits 7031 and 7032, alogic circuit (NAND gate) 704, a row address buffer 721, a row decoder722, a column address buffer 723, a column decoder 724, a data I/Obuffer (multiplexer) 725, write circuits 7261 and 7262, sense amplifiers7271 and 7272, and source voltage supply circuits 7091 and 7092. Thesemiconductor memory device also includes bit lines BLs, word lines WLs,memory cells MCs, a write control signal W that becomes high level Hwhen writing data, and an erase control signal E that becomes high levelH when erasing data. Compared with the conventional semiconductor memorydevice of FIG. 51, the embodiment of FIG. 52 additionally has theexpected value storage circuits 7021 and 7022, coincidence circuits 7031and 7032, multiplexer (data I/O buffer) 725, and NAND gate 704.

To erase blocks of memory cells, the source voltage supply circuits 7091and 7092 latch a selection signal of the block address buffer 701 when alatch control signal LT becomes high level H. The erase control signal Eis set to high level H to activate all the source voltage supplycircuits that have latched the selection signal, thereby simultaneouslyerasing blocks of memory cells.

FIGS. 53 to 55 show examples of the source voltage supply circuit 7091(7092), expected value storage circuit 7021 (7022), and coincidencecircuit (7031 (7032) of the semiconductor memory device of FIG. 52,respectively.

In FIG. 53, the source voltage supply circuit 7091 (7092) has a NANDgate 731 for receiving the block address signal (block selection signal)and latch control signal LT, a latch circuit formed of a NAND gate 732and an inverter 733, a NAND gate 734 for receiving an output of thelatch circuit as well as the erase control signal E, p-channel type MOStransistors 736 and 737 connected to an erase power source Vpp, and ann-channel type MOS transistor 738. In FIG. 54, the expected valuestorage circuit 7021 (7022) includes inverters 741, 744, 745, 746, and750, and NAND gates 742, 743, 747, 748, and 749. The NAND gate 743 andinverter 744 form a latch circuit. An output of the latch circuit iscontrolled in response to an inversion control signal INV. In FIG. 55,the coincidence circuit 7031 (7032) has an inverter 753, NAND gates 751,752, and 755, and an exclusive OR gate 754. The NAND gate 752 andinverter 753 form a latch circuit, which determines whether or not anoutput of the sense amplifier 7272 (7272) agrees with a reference outputof the expected value storage circuit 7021 (7022).

In the above arrangements, an erase operation is carried out by latchingthe block address signal. In this case, only the blocks that havelatched the block address signal are erased with the erase controlsignal E. To carry out a write-before-erase operation, the latch controlsignal LT is set to high level H, and write data “0” is latched by theexpected value storage circuit 7021 (7022). Namely, the data I/O buffer725 transfers data “0” to the expected value storage circuit 7021 (7022)selected by the block address signal, and the circuit 7021 (7022)latches the data “0” when the latch control signal LT becomes high levelH. An output of the coincidence circuit 7031 (7032) of an unselectedblock is forcibly set to high level H. When the write control signal Wbecomes high level H, the data is written to the selected block(s).

To carry out a verify operation, the coincidence circuit 7031 (7032)compares the expected value stored in the expected value storage circuit7021 (7022) with an output of the sense amplifier 7271 (7272). Outputsof the coincidence circuits 7031 and 7032 are sent to the NAND gate 704.If the data has been sufficiently written, the sense amplifier 7271(7272) provides a low level output, and therefore, the coincidencecircuit 7031 (7032) provides a high level output. If the data has beensufficiently written to all the selected blocks, the NAND gate 704provides a verify output VER of low level L to confirm that the data hasbeen successfully written to a given address of each selected block. Tocarry out an erase verify operation, the expected value is invertedaccording to the inversion control signal INV and is verified. If dataat a given address is “1” in every selected block, the verify output VERwill be low level similar to the write operation, to confirm that datahas been erased. In this way, according to the semiconductor memorydevice of this embodiment, the output of the NAND gate 704 becomes lowlevel L only when all outputs of the coincidence circuits 7031 and 7032are each at high level H. If any one of the coincidence circuitsprovides a low level output, it is confirmed that there is a defectivebit.

FIG. 56 is a circuit diagram showing a semiconductor memory deviceaccording to another embodiment of the seventh aspect of the presentinvention. This embodiment employs an expected value generator 7041(7042) instead of the expected value storage circuit 7021 (7022) of FIG.52.

The semiconductor memory device of FIG. 52 may be realized without aunit for storing random data because reference data necessary for thewrite-before-erase and erase operations are “0” and “1” for all bits.According to the embodiment of FIG. 56, the expected value generator7041 (7042) latches its block address signal (selection signal) when thelatch control signal LT becomes high level H. The expected valuegenerator 7041 (7042) that has latched the selection signal forciblyprovides data of “0.” When carrying out an erase verify operation, theexpected value is inverted in response to the inversion control signalINV.

FIG. 57 shows an example of the expected value generator 7041 (7042) ofFIG. 56. The expected value generator has inverters 763, 764, 767, and768, NAND gates 761, 762, 766, 769, and 770, and a NOR gate 765.

The NAND gate 762 and inverter 763 form a latch circuit. This expectedvalue generator forcibly sets reference data to low level L when thelatch circuit (762, 763) latches an address data signal (block selectionsignal), and when the inversion control signal INV becomes high level H,changes the reference data to high level H.

FIG. 58 is a circuit diagram showing a semiconductor memory deviceaccording to another embodiment of the seventh aspect of the presentinvention. This embodiment employs a block selection signal storagecircuit 7051 (7052), a write circuit 7161 (7162), and a data inversioncircuit 7061 (7062), in place of the expected value storage circuit 7021(7022), write circuit 7261 (7262), and coincidence circuit 7031 (7032)of FIG. 52. Namely, this embodiment controls the write-before-eraseoperation, verify operation thereof, and erase verify operation inresponse to a selection signal stored in the storage circuit 7051(7052).

The semiconductor memory device of FIG. 58 carries out thewrite-before-erase operation by writing data to cells in a blockspecified by a block selection signal (a block address signal). The datainversion circuit 7061 (7062) inverts data of the sense amplifier 7271(7272) between the erase verify operation and the write verifyoperation, and provides an output of high level H if the write or eraseoperation has been sufficiently done. At this time, the sense amplifier7271 (7272) of an unselected block always provides an output of highlevel H due to an output signal of the storage circuit 7051 (7052).Consequently, the write-before-erase operation and erase operation areachieved as explained before.

FIGS. 59 to 61 show examples of the block selection signal storagecircuit 7051 (7052), write circuit 7161 (7162), and data inversioncircuit 7061 (7062) of the semiconductor memory device of FIG. 58,respectively.

In FIG. 59, the block selection signal storage circuit 7051 (7052) has aNAND gate 771 for receiving the block address signal (block selectionsignal) and latch control signal LT, and a latch circuit formed of aNAND gate 772 and an inverter 773. In FIG. 60, the write circuit 7161(7162) has an inverter 781, a NOR gate 782, a NAND gate 783, p-channeltype MOS transistors 785 and 786 for receiving a write power source Vpp,and an n-channel type MOS transistor 787. In FIG. 61, the data inversioncircuit 7061 (7062) has inverters 792 and 793, and NAND gates 791, 794,795, and 796. The data inversion circuit inverts an output of the senseamplifier 7271 (7272) in response to the inversion signal INV andsupplies the inverted signal to the NAND gate 704.

The block selection signal storage circuit 7051 (7052) of FIG. 59latches the block address signal in the latch circuit (772, 773) inresponse to the latch control signal LT of high level H. In the blockthat has latched the block selection signal, the write voltage Vpp isapplied to a bus when the write control signal W is set to high level H,irrespective of the level of input data.

The data inversion circuit 7061 (7062) of FIG. 61 inverts the output ofthe sense amplifier 7271 (7272) to low level L (a written state) whenthe inversion control signal INV becomes low level L in the write verifyoperation, and provides an output of high level H to the NAND gate 704.In the erase verify operation, the inversion control signal INV is setto high level H. In an unselected block, the block selection signal(block address signal) is at low level L, and therefore, the output tothe NAND gate 704 is forcibly set to high level H. As a result, optionalblocks are simultaneously erased. The multiplexer 725 follows the blockselection address to supply write data to given blocks and data fromgiven sense amplifiers to the outside.

As explained above, the semiconductor memory device according to theseventh aspect of the present invention employs a latch circuit of ablock selection signal in a source voltage supply circuit of each cellblock. The source voltage supply circuits are simultaneously activated.Each cell block is provided with a sense amplifier, an expected valuegenerator, and a circuit for confirming the coincidence of a senseamplifier output and an expected value. The semiconductor memory devicefurther has a circuit for providing an AND output of the coincidencecircuits, to simultaneously carry out erase and verify operations.

A semiconductor memory device according to an eighth aspect of thepresent invention will be explained with reference to FIGS. 62 to 73.

Semiconductor memories such as EEPROMs (electrically erasableprogrammable read only memory) have attracted attention recent years. Inparticular, flash memories have attracted considerable attention as areplacement for magnetic storage devices because they are suitable forintegration. Although the following explanation of the present inventionis made in connection with the flash memories, the present invention isalso applicable for standard EEPROMs.

Each memory cell of a flash memory has a floating gate and a controlgate. Data is stored in the cell depending on whether or not thefloating gate holds charges. To write data to the cell, a high voltageof about +12 V is applied to the control gate, a voltage of about +6 Vis applied to a drain, a source is grounded, and hot electrons areinjected into the floating gate. To read data from the cell, a voltageof about +5 V is applied to the control gate, a voltage of about +1 V isapplied to the drain, the source is grounded, and charges accumulated inthe floating gate are sensed to detect a current difference. To erasedata from the cell, a high voltage of about +12 V is applied to thesource, the control gate is grounded, and the drain is opened to producea tunnel current between bands. As a result, electrons are drawn fromthe floating gate to the source. When writing data to the cell, a draincurrent of about several milliamperes flows, and a current of severaltens of milliamperes is needed in total. Accordingly, a power source fora normal voltage Vcc (+5 V) and a power source for a high voltage Vpp(+12 V) are needed.

FIG. 62 shows a conventional computer system employing a flash memory.In addition to a power source for a normal voltage, this system has apower source for a high voltage. The system involves a centralprocessing unit (CPU) 821, a ROM/RAM 822, an input/output (I/O) port823, a peripheral device 824, the flash memory 825, the high voltagesupply source 828, and a control port 829 for controlling the highvoltage supply source 828. The high voltage supply source 828 includes ahigh voltage controller 826 and a high voltage generator 827. The highvoltage supply source 828 is controlled by the CPU 821 through thecontrol port 829, to supply and stop a high voltage Vpp to the flashmemory 825. The high voltage generator 827 may be a fixed power sourceor a step-up circuit. In the case of the fixed power source, the highvoltage controller 826 will be simply a switch. In the case of thestep-up circuit, it is activated by the controller 826 only when needed,to reduce power consumption.

The flash memory 825 is not frequently written to or erased, andtherefore, the high voltage is supplied only when needed. FIG. 63 showssteps of controlling the high voltage supply source 828 when writing anderasing data in the system of FIG. 62. Each of the steps will be brieflyexplained.

Step S831 provides the control port 829 with an instruction to supply ahigh voltage. In the case of the fixed power source, the controller 826is switched to supply the high voltage. In the case of the step-upcircuit, a step-up operation is started. Step S832 waits until asufficient high voltage is supplied. In the case of the fixed powersource, this step is omitted.

Step S833 writes a write command or an erase command to the flash memory825. Steps S834 and S835 test a state after the step S833. If no problemis found, step S836 causes the control port 829 to provide a signal forstopping the high voltage.

In practice, it is disadvantageous to prepare a high voltage supplysource in addition to a normal voltage supply source. Namely, flashmemories operating with a single power source are needed.

One of such flash memories operating with a single power sourceincorporates a step-up circuit for increasing a normal voltage Vcc to ahigh voltage Vpp. The increased high voltage Vpp is applied to thecontrol gate of a memory cell, and the normal voltage Vcc instead of avoltage of +6 V is applied to the drain thereof. Although this techniquelowers the avalanche breakdown voltage of the drain, it causes littlecurrent to flow to the control gate so that even the internal step-upcircuit having a small current supply capacity will be sufficient toproduce the voltages. When a voltage is applied to the drain, a largequantity of current is needed. In this case, the normal voltage is used.Consequently, the single power source works to write and erase data.

To reduce the power consumption of the flash memory, it is necessary toreduce the normal voltage Vcc. When the normal voltage Vcc, which islow, is directly applied to the drain of a memory cell, the normalvoltage Vcc may be lower than the avalanche breakdown voltage. In thiscase, data cannot be written. To solve this problem, a separate highvoltage source similar to that of the prior art may be used. Manysystems, however, are designed to work with a single power source, sothat, if a flash memory operating with a separate high voltage source isemployed for such systems, software for controlling the high voltagesource must be modified. This modification is troublesome thusdeteriorating the value of the flash memory.

Since the flash memories do not work with a low voltage, the normalvoltage Vcc must be high. This means that a power source providing a lownormal voltage is useless for the flash memories.

An object of the eighth aspect of the present invention is to provide asemiconductor memory device employing two power sources that are easy touse and operable like a single power source.

FIG. 64 explains the principle of an electrically erasable nonvolatilesemiconductor memory device according to the eighth aspect of thepresent invention.

This semiconductor memory device (nonvolatile semiconductor memorydevice) writes or erases data with a high voltage Vpp that is higherthan a normal voltage Vcc used to read data. The nonvolatilesemiconductor memory device has a terminal 806 for receiving the highvoltage Vpp from an external high voltage supply unit 802, in additionto a terminal for receiving the normal voltage. The semiconductor memorydevice has a control terminal 807 for providing the high voltage supplyunit 802 with a control signal to control the supply of the high voltageVpp.

The signal terminal 807 of the semiconductor memory device (EEPROM)according to the eighth aspect of the present invention provides, whenneeded, the high voltage supply unit 802 with a control signal tocontrol the supply of the high voltage. Unlike the prior art, a computersystem employing the flash memory of the present invention is notrequired to control the high voltage supply unit 802. Similar to thenonvolatile semiconductor memory device having a single power source,the semiconductor memory device of the present invention automaticallycontrols the high voltage supply unit 802 when the semiconductor memorydevice is accessed. Accordingly, the nonvolatile semiconductor memorydevice of the eighth aspect of the present invention is operable in thesame manner as that with a single power source, and requires no softwareamendment.

FIG. 65 shows steps of providing a high voltage in a system employingthe nonvolatile semiconductor memory device (EEPROM) of the presentinvention.

Steps S801 to S804, on the left side of FIG. 65, are carried out in thesystem, and steps S810 to S813 are carried out in the EEPROM. Step S801sends a write or erase command to the EEPROM. In response to command,step S810 determines the kind of the command. If it is a command to readdata, no high voltage is required. In this case, the following steps areskipped, and a read operation is carried out. If the command is to writeor erase data, step S811 causes the signal terminal 807 to send aninstruction to start the supply of the high voltage. Then, the processwaits until the supplied high voltage reaches a sufficient level. If thehigh voltage power source is a fixed power source, no wait is required.

Once the supplied high voltage reaches a sufficient level, step S812carries out the write or erase operation. In the case of the eraseoperation, the system is not required to carry out any of the operation,and therefore, the system can commence another operation. In the case ofthe write operation, the system provides write data. In any case, in themiddle of or after the write or erase operation, the system carries outa test in steps S802 and S803.

Step S804 informs the EEPROM of the completion of the write or eraseoperation. In response to this, the EEPROM sends a signal to stop thehigh voltage in step S813. The write or erase operation usually handlesa large quantity of data, so that the write/erase operation continuesfor a certain period. Accordingly, if the EEPROM detects that thewrite/erase operation has ended in the middle of a certain period, itmay automatically provide the signal to stop the high voltage.

In any case, processes to be carried out in the system are simplifiedcompared with the prior art of FIG. 63.

FIG. 66 shows a system according to an embodiment of the semiconductormemory device according to the eighth aspect of the present invention.

This system includes a central processing unit (CPU) 831, a ROM/RAM 832,an I/O port 833, a peripheral device 834, a flash memory 835 of thepresent invention, and a DC-DC converter 836 for generating a highvoltage. The DC-DC converter 836 is turned ON and OFF in response to acontrol signal from the flash memory 835.

In comparison with the system of FIG. 62, the system of FIG. 66 has nooutput port through which the CPU 831 controls the DC-DC converter 836.The system of FIG. 66 is basically the same as a system with a flashmemory having a single power source.

FIG. 67 shows the internal arrangement of the flash memory 835 of FIG.66. The flash memory 835 has an address buffer 841, a row decoder 842, acolumn decoder 843, a memory cell matrix 844, a read/write amplifier845, an I/O buffer 846, and a controller 847. The flash memory 835involves external I/O terminals such as an address terminal, a dataterminal, a normal power source (Vcc) terminal, a high voltage (Vpp)terminal, and a control terminal. These are the same as those of theconventional flash memory.

What is different from the conventional flash memory is that the flashmemory of FIG. 67 has a command determination unit 849, a voltage testcircuit 850, and an external I/O terminal for controlling a high voltagesupply source. The command determination unit 849 is a comparator forfinding, for example, a write or erase command to the flash memory 835.When the CPU 831 provides a write or erase command to the flash memory835, the command determination unit 849 detects the command and providesthe high voltage source control terminal with a signal to start thesupply of the high voltage. The command determination unit 849 may be atimer circuit that is reset by a write signal *WE.

The voltage test circuit 850 tests if a supplied high voltage is greaterthan a predetermined value. FIG. 69 shows an example of the voltage testcircuit 850. This circuit tests not only the high voltage but also ifthe high voltage Vpp and normal voltage Vcc applied to internal powersource lines are greater than respective reference values. The voltagetest circuit 850 includes a sequence circuit 861 for carrying out a maincontrol operation. A comparator 862 compares a voltage, which comes froman internal power source line 864 and is divided by a resistor 865, withone of reference voltages r₁ and r₂. These reference voltages areswitched from one to another through a switch 863.

FIG. 68 shows steps of controlling the external power source for theflash memory 835. Step S821 checks a signal provided by the CPU 831 tothe flash memory 835, to see whether or not a high voltage is required.If the high voltage is required, step S822 sends an activation signal tothe DC-DC converter 836. The DC-DC converter 836 needs a certain periodafter the activation thereof until it provides the required voltage.Accordingly, there is a wait time until the voltage test circuit 850confirms in step S823 that the voltage of the internal power source lineis greater than the predetermined value.

When the voltage becomes greater than the predetermined value, step S824carries out a write or erase operation. This operation continues for awhile. After the write or erase operation is complete, step S825 checksto see if data has been correctly written or erased. If the result isYES, step S826 sends a stop signal to the DC-DC converter 836. At thesame time, the internal power source is connected. The DC-DC converter836 then stops supplying the high voltage. Step S827 confirms that theconnected internal power source is restored to a normal voltage, and theprocess ends.

In FIG. 68, the voltage test circuit 850 of FIG. 67 is used to test thevoltage provided by the DC-DC converter 836. It is also possible tomeasure, in advance, a time after the activation of the DC-DC converter836 until the predetermined voltage is provided, and an operation thatrequires the high voltage is started after the measured time.

FIG. 70 shows an example of such a technique. Upon detecting anoperation that requires the high voltage, a controller 871 provides thehigh voltage control terminal with a signal to activate the DC-DCconverter 836, and temporarily stops working. The activation signal isalso provided to a delay circuit 872, which provides the controller 871with a delayed signal after a predetermined time. In response to thesignal, the controller 871 restarts.

The control operation of FIG. 68 may be realized by a microcomputer. Itis not practical, however, to install the microcomputer in the flashmemory 835. The embodiment mentioned above, therefore, employs the delaycircuit and logic circuits to achieve the control operation.

Although the DC-DC converter 836 is disposed outside the flash memory835 in the embodiment, it may be incorporated in the flash memory withthe same control mechanism explained above. The DC-DC converter 836 willbe explained next.

FIGS. 71 and 72 show examples of DC-DC converters. The DC-DC converter881 of FIG. 71 employs an inductance element 886 and includes anoscillator 882 and a switch 883 for controlling a step-up operation. Aswitch 884 is turned ON and OFF in response to a signal from theoscillator 882. The inductance element 886 is a coil connected between adiode 885 and the normal power source Vcc. When the switch 884 is turnedON and OFF, the amplitude of a voltage at an input end of the diode 885increases according to the principle of a transformer. An output end ofthe diode 885 provides only high voltage components, thereby providing ahigh voltage output. Switching the switch 883 controls the turning ONand OFF of the switch 884, to control an increase in voltage. Componentsexcept the coil 886 are relatively compact, so that they may beincorporated in the flash memory 835. The coil 886 is difficult toreduce in size without deteriorating performance. When incorporating theDC-DC converter 836 in the flash memory 835, it is preferable,therefore, to arrange the coil 886 outside the flash memory 835.

The DC-DC converter 891 of FIG. 72 employs a capacitor 894 as a passivecomponent. An increase in voltage is controlled by controlling anoscillator 892. It is also difficult to install the capacitor 894 in theflash memory 835. When incorporating the DC-DC converter 891 in theflash memory 835, the capacitor 894 may be arranged outside the flashmemory 835.

FIG. 73 shows a package in which a flash memory 812 and a DC-DCconverter 813 are mounted on an element to which a passive component 814such as an inductance element or a capacitance element is attached.

A semiconductor memory device according to the ninth aspect of thepresent invention will be explained next with reference to FIGS. 74 to91.

Generally, semiconductor memory devices (for example, EEPROMs, flashmemories) must be tested in a production line, and this testing isparticularly important for new products to analyze and guarantee them.To easily carry out the test, the semiconductor memory devices usuallyincorporate test functions such as full selection and nonselectionfunctions of word or bit lines. These functions are provided for adecoder circuit in the semiconductor memory devices, and are used whenthe test is carried out in the production line.

FIGS. 74 to 77 each show an example of a decoder arranged in aconventional decoder circuit of a semiconductor memory device. In FIG.74, the decoder has a NOR output circuit composed of CMOS transistors.The decoder includes p-channel type MOS transistors TRP1, TRP2, andTRP3, and n-channel type MOS transistors TRN3, TRN4, and TRN6. A sourceof the transistor TRP1 is connected to a high voltage supply source Vcc,and a source of the transistor TRN6 is connected to a low voltage supplysource Vss.

As shown in FIG. 74, a gate of the transistor TRP1 is connected to agate of the transistor TRN3, and an output of an address buffer AD1 issupplied to these gates of the transistors TRP1 and TRN3. A source ofthe transistor TRP5 is connected to the high voltage supply source Vcc,and a drain of thereof is connected to an output end VOT of the decoder.A full selection control signal AH is supplied to gates of thetransistors TRP5 and TRN6, and a nonselection signal /An is supplied togates of the transistors TRP2 and TRN4. Note, a mark “/” denotes aninverted signal, and thus the signal /An denotes an inverted signal ofthe signal An. Further, the signal /An may be a specific terminal forreceiving a specific high signal (extremely high voltage signal), or thesignal /An may be a terminal for receiving a specific address signal.

This decoder of FIG. 74 usually achieves a decoder function only withthe NOR circuit, and to achieve a test function, the decoder needs thetransistors TRP5 and TRN6 as control transistors in addition to thebasic decoder circuit. These transistors are set to high or lowpotential in response to a control signal, to carry out a full selectionoperation or a nonselection operation.

Note, in FIG. 74, the full selection control signal AH controls thegates of the transistors TRP5 and TRN6, to carry out the full selectionor nonselection operation. To carry out the full selection operation,the full selection control signal AH is set to low level L to turn ONthe transistor TRP5, thereby maintaining a high potential on a word orbit line. On the other hand, to carry out the nonselection operation,the full selection and nonselection control signals AH and /An are eachset to high level H to turn ON the transistors TRN4 and TRN6, therebydropping the potential of the word or bit line.

This conventional arrangement increases the size and cost of the decodercircuit. When semiconductor memory devices are more integrated, decoderlayout pitches must be narrower making it difficult to achieve the testfunction.

The decoder of FIG. 75 resembles that of FIG. 74. In FIG. 74, a decodingsection is a NOR circuit composed of CMOS transistors. FIG. 75 differsfrom FIG. 74 in that FIG. 75 employs a full selection signal An insteadof the total nonselection signal /An, and an inverter INV1 is connectedto the output end of the decoder. Namely, the logic of FIG. 75 isopposite to that of FIG. 74.

The decoder of FIG. 76 differs from the decoder of FIG. 74 in that ithas an output section composed of a NAND circuit of CMOS transistors.The decoder of FIG. 76 includes p-channel type MOS transistors TRP11,TRP12, and TRP13, and n-channel type MOS transistors TRN14, TRN15, andTRN16. A source of the transistor TRP11 is connected to a high voltagesupply source Vcc, and a source of the transistor TRN16 is connected toa low voltage supply source Vss. A source of the transistor TRN15 isconnected to the low voltage supply source Vss, and a drain thereof isconnected to an output end of the decoder. A gate of the transistorTRP12 is connected to a gate of the transistor TRN14, and a gate of thetransistor TRN14 is connected to an output of an address buffer AD2. Aseparate nonselection control signal AL is supplied to gates of thetransistors TRP11 and TRN15, and a full selection signal An is suppliedto gates of the transistors TRP13 and TRN16.

When the nonselection control signal AL is set to high level H, thetransistor TRN15 is turned ON to drop the output of the decoder, therebyputting a word or bit line in an unselected state. When the nonselectionand full selection control signals AL and An are each set to low levelL, the, transistors TRP11 and TRP13 are turned ON. As a result, thedecoder provides the high voltage Vcc to carry out full selection.

The decoder of FIG. 77 resembles that of FIG. 76. A decoding section ofFIG. 77 is a NAND circuit of CMOS transistors. FIG. 77 differs from FIG.76 in that it employs a nonselection signal /An instead of the fullselection signal An, and an inverter INV2 is connected to an output endof the decoder. Namely, the logic of FIG. 77 is opposite to that of FIG.76. All of these conventional decoders has the problems mentionedbefore.

An object of the ninth aspect of the semiconductor memory device of thepresent invention is to provide an inexpensive decoder circuit thatsolves the problems of the prior art. The decoder circuit of the presentinvention is simple and compact to achieve the full selection andnonselection of word or bit lines in a test mode. This decoder circuitis suitable for high integration.

FIGS. 78 to 81B show embodiments of a decoder circuit of a semiconductormemory device according to the ninth aspect of the present invention.

As shown in FIG. 78, an output row or a decoding row of the decodercircuit 901 is connected to a first power source 904 for supplying ahigh voltage Vcc and to a second power source 905 for supplying a lowvoltage (ground voltage) vss or the high voltage Vcc in response to acontrol signal AH. The output row of the decoder circuit 901 is composedof decoders DEC1 to DECn. Namely, each of the decoders DEC1 to DECn isconnected to the first power source 904 for supplying the high voltageVcc, and each of the decoders DEC1 to DECn receives address signals A0to Am and /A0 to /Am. The decoders DEC1 to DECn (902) have output endsVOT1 to VOTn, respectively.

In response to the control signal AH, the second power source 905supplies the low voltage Vss or the high voltage Vcc. The second powersource 905 is shared by the decoders DEC1 to DECn. Namely, an output endof the second power source 905 is connected to input ends of thedecoders DEC1 to DECn. A nonselection signal /An is supplied to an inputend of each of the decoders DEC1 to DECn.

Note, in the ninth aspect of the present invention, the second powersource 905 may have any circuit arrangement if it can provide thefunction mentioned above. For example, the second power source 905 mayhave an inverter 906 as shown in FIG. 78. In response to the level ofthe control signal AH, the inverter 906 provides the high voltage Vcc orthe low voltage Vss.

The operation of the decoder circuit of FIG. 78 will be explained next.To carry out a nonselection operation, the nonselection signal /An andcontrol signal AH are each set to high level H. The inverter 906provides the voltage Vss of low level L, and the decoders DEC1 to DECnconnected to the inverter 906 operate like conventional decoders. Theoutput ends VOT1 to VOTn of the decoders DEC1 to DECn are each at lowlevel L to establish a nonselection state. On the other hand, to carryout a full selection operation, the nonselection signal /An is set tohigh level H, and the control signal AH to low level L. The inverter 906provides the voltage Vcc of high level H, and the decoders DEC1 to DECnconnected to the inverter 906 receive the voltage Vcc, and the outputends VOT1 to VOTn of the decoders DEC1 to DECn each provide the voltageVcc of high level H to establish a full selection state.

In this way, the single second power source 905 is shared by thedecoders DEC1 to DECn, and therefore, no additional circuits nortransistors are required for the decoder circuit. This results insuppressing the size of the decoder circuit and easily carrying out thefull selection or nonselection operation of word or bit lines in a testmode.

The decoder circuit of FIG. 79 is basically the same as that of FIG. 78and is characterized by a decoding row 903 (dec1 to decn). Each of thedecoders is connected to a first power source 904 for supplying a highvoltage Vcc, and receives decoder selection signals A0 to Am and /A0 to/Am. Output ends N1 to Nn of the decoders dec1 to decn are connected tooutput terminals VOT1 to VOTn through inverters INV7-1 to INV7-n,respectively.

The decoders dec1 to decn share a second power source 905, whichsupplies a low (ground) voltage Vss or the high voltage Vcc in responseto a control signal AL. Namely, an output end of the second power source905 is connected to an input of each of the decoders dec1 to decn. Atotal selection signal An is supplied to an input of each of thedecoders dec1 to decn. The second power source 905 has an inverter 906.In response to the level of the control signal AL, the inverter 906provides the high voltage Vcc or the low voltage Vss.

The operation of FIG. 79 will be explained next. To carry out a fullselection operation, the full selection signal An is set to high levelH, and the control signal AL to high level H. The inverter 906 providesthe voltage Vss of low level L, and the decoders dec1 to decn connectedto the inverter 906 operate like the conventional decoders. The outputends N1 to Nn of the decoders dec1 to decn each provide the voltage Vssof low level L. The low level signals are inverted by the invertersINV7-1 to INV7-n, so that the output ends VOT1 to VOTn each provide highlevel H, to establish a full selection state. On the other hand, toachieve a nonselection operation, the full selection signal An is set tohigh level H, and the control signal AL is set to low level L. Theinverter 906 provides the voltage Vcc of high level H. Accordingly, thedecoders dec1 to decn connected to the inverter 6 receive the voltageVcc, and the output ends N1 to Nn of the decoders dec1 to decn eachprovide the voltage Vcc of high level H. These high level outputs areinverted by the inverters INV7-1 to INV7-n, so that the output ends VOT1to VOTn each become low level L to establish a nonselection state.

The decoder circuit of FIG. 80A is basically the same as that of FIG. 78but inverts the polarities of signals.

The decoder circuit 901 of FIG. 80A has an output row of decoders DEC1to DECn. Each of the decoders DEC1 to DECn is connected to a first powersource 904 for supplying a low voltage Vss, and receives address signalsA0 to Am and /A0 to /Am. The decoders have output ends VOT1 to VOTn,respectively. In response to a control signal AL, the second powersource 905 supplies the low voltage Vss or the high voltage Vcc. Thesecond power source 905 is shared by the decoders DEC1 to DECn. Namely,an output end of the second power source 905 is connected to an inputend of each of the decoders DEC1 to DECn. A full selection signal An issupplied to each of the decoders DEC1 to DECn. The second power source905 has an inverter 906, and the inverter 906 provides the high voltageVcc or the low voltage Vss in response to the level of the controlsignal AL.

The operation of the embodiment of FIG. 80A will be explained next. Tocarry out a full selection operation, the full selection signal An andcontrol signal AL are each set to low level L. The inverter 906 providesthe voltage Vcc of high level H, and the decoders DEC1 to DECn connectedto the inverter 906 operate like conventional decoders. The output endsVOT1 to VOTn of the decoders DEC1 to DECn each become high level H toestablish a full selection state. On the other hand, to carry out anonselection operation, the full selection signal An is set to low levelL, and the control signal AL is set to high level H. The inverter 906provides the voltage Vss of low level L. Accordingly, the decoders DEC1to DECn connected to the inverter 906 receive the voltage Vss, and theoutput ends VOT1 to VOTn each become low level L to establish anonselection state.

FIG. 80B shows a modification of the decoder circuit shown in FIG. 80A.

As shown in FIG. 80B, in this modification, two second power sources 905a and 905 b are provided for the decoder circuit 901 having an outputrow of decoders DEC1 to DECn. Namely, the power source 905 a is providedfor the decoders DEC1 to DECk, and the power source 905 b is providedfor the decoders DECk+1 to DECn. For example, the total number of thedecoders DEC1 to DECn is specified as 1026 or 512, and each of the powersources 905 a and 905 b is provided for 512 or 256 decoders DEC1 to DECkand DECk+1 to DECn, respectively. Note, the control signal AL applied toinverters 906 a and 906 b of the power sources 905 a and 905 b is thesame signal. Further, the number of the power sources is not limited totwo, but can be specified as four or more. In this case, the decodersDEC1 to DECn are divided into four or more plural groups, and each ofthe power sources is provided for each of the divided groups,respectively. In this modification shown in FIG. 80B, transistorsconstituting the inverters 906 a and 906 b can be formed in a smallersize than that of the inverter 906 shown in FIG. 80A.

The decoder circuit of FIG. 81A is basically the same as that of FIG.80A and is characterized by a decoding row 903.

The decoding row 903 includes decoders dec1 to decn. Each of thesedecoders is connected to a first power source 904 for supplying a lowvoltage Vss, and receive decoder selection signals A0 to Am and /A0 to/Am. Output ends N1 to Nn of the decoders dec1 to decn are connected tooutput terminals VOT1 to VOTn through inverters INV7-1 to INV7-n,respectively.

The decoders dec1 to decn share a second power source 905, whichsupplies the low voltage Vss or a high voltage Vcc in response to acontrol signal AH. Namely, an output end of the second power source 905is connected to an input end of each of the decoders decl to decn. Anonselection signal /An is supplied to each of the decoders dec1 todecn. The second power source 905 has an inverter 906. In response tothe control signal AH, the inverter 906 provides the high voltage Vcc orthe low voltage Vss.

The operation of the embodiment of FIG. 81A will be explained. Toestablish a nonselection state, the nonselection signal /An and controlsignal AH are each set to low level L.

The inverter 906 provides the voltage Vcc of high level H, and thedecoders dec1 to decn connected to the inverter 906 operate likeconventional decoders. The output ends N1 to Nn of the decoders eachprovide the voltage Vcc of high level H. These high-level outputs areinverted by the inverters INV7-1 to INV7-n, and the output terminalsVOT1 to VOTn each provide low level L to establish the nonselectionstate. On the other hand, to establish a full selection state, thenonselection signal /An is set to low level L and the control signal AHto high level H. The inverter 906 provides the voltage Vss of low levelL. Accordingly, the decoders dec1 to decn connected to the inverter 906each receive the voltage Vss, and the output ends N1 to Nn of thedecoders each provide the voltage Vss of low level L. These low-leveloutputs are inverted by the inverters INV7-1 to INV7-n, and the outputterminals VOT1 to VOTn each provide high level H to establish the fullselection state.

In this way, the decoders DEC1 to DECn share the single second powersource 905. Namely, the decoder circuit according to the presentinvention requires no additional circuits or transistors. This resultsin realization of a compact decoder circuit that easily carries out afull selection or nonselection operation of word or bit lines.

FIG. 81B shows a modification of the decoder circuit shown in FIG. 81A.

As shown in FIG. 81B, in this modification, two second power sources 905a and 905 b are provided for the decoder circuit 901 having an outputrow of decoders dec1 to decn. Namely, the power source 905 a is providedfor the decoders dec1 to deck, and the power source 905 b is providedfor the decoders deck+1 to decn. For example, the total number of thedecoders dec1 to decn is specified as 1026 or 512, and each of the powersources 905 a and 905 b is provided for 512 or 256 decoders dec1 to deckand deck+1 to decn, respectively. Note, similar to the modification ofFIG. 80B, the control signal AH applied to inverters 906 a and 906 b ofthe power sources 905 a and 905 b is the same signal. Further, thenumber of the power sources is not limited to two, but can be specifiedas four or more plural. In this case, the decoders DEC1 to DECn aredivided into four or more plural groups, and each of the power sourcesis provided for each of the divided groups, respectively. In thismodification shown in FIG. 81B, transistors constituting the inverter906 a and 906 b can be formed in a smaller size than that of theinverter 906 shown in FIG. 81A.

FIG. 82 shows an example of a semiconductor memory device (flash memory)using a decoder circuit according to the ninth aspect of the presentinvention. In FIG. 82, reference 912 denotes a row decoder, 914 denotesa column decoder, 917 denotes a sense amplifier, and 919 denotes asource voltage supply circuit. Notes in FIG. 82, the row decoder 912includes a row address buffer, and the column decoder 914 includes acolumn address buffer.

As shown in FIG. 82, the semiconductor memory device comprises the rowdecoder 912, the column decoder 914, a sense amplifier 917, and thesource voltage supply circuit 919. Further, the semiconductor memorydevice comprises memory cells (cell transistor) MCs each made of ann-channel type MOS (MIS) transistor, word lines WLs, bit lines BLs, andsource lines SLs. The source voltage supply circuit 519 is connected tothe sources of the memory cells MCs in a memory cell array through thesource lines SLs, to electrically collectively erase the memory cells.The decoder circuit according to the ninth aspect of the presentinvention is applied to the row decoder 912 or column decoder 914. Note,the decoder circuit of the ninth aspect of the present invention notonly has a normal decoding function, but also has a test function tocarry out a full selection or nonselection operation of word or bitlines.

FIGS. 83A and 83B show the details of the decoder circuit of FIG. 78,and FIG. 83B shows transistors forming the decoder circuit.

In FIG. 83A, the decoder circuit 901 has the output row 902 involvingthe decoders DEC1 to DECn. Each of the decoders DEC1 to DECn has a NANDgate 920 for receiving the address signals AO to Am and /A0 to /Am, anda NOR gate 921 for receiving an output of the NAND gate 920 and thenonselection signal /An. An output of the NOR gate 921 is connected to acorresponding one of the output terminals VOT1 to VOTn. Otherarrangements and operation of FIG. 83A are the same as those of FIG. 78.

FIG. 83B shows transistors forming the elements of FIG. 83A. The NANDgate 920 has p-channel type MOS transistors TRP91 and TRP92, andn-channel type MOS transistors TRN931 to TRN93n. These transistors aredisposed between the high voltage supply source Vcc and the low voltagesupply source Vss. The NOR gate 921 may be a known NOR gate such as theone shown in FIG. 74. The inverter 906 may be a known CMOS inverter(transistors TRP906 and TRN906).

The operation of the decoder circuit of FIGS. 83A and 83B is the same asthat of the decoder circuit of FIG. 78, and therefore, will not beexplained again.

FIGS. 84A and 84B show the details of the decoder circuit of FIG. 79, inwhich FIG. 84B shows transistors forming the decoder circuit.

In FIG. 84A, the decoder circuit 901 has the decoder row 903 involvingthe decoders dec1 to decn. The decoder circuit also has the invertersINV7-1 to INV7-n that are not provided for the decoder circuit of FIG.83A. The operation of the decoder circuit of FIGS. 84A and 84B is thesame as that of the decoder circuit of FIG. 79, and therefore, will notbe explained again.

FIGS. 85A and 85B show the gates and transistors of the decoder circuitof FIG. 80A.

The decoder circuit 901 of FIGS. 85A and 85B differ from those of FIGS.83A, 83B and 84A, 84B in that each decoder of the decoder circuit 901 isformed of NAND gates 920 and 920′. Note, the NAND gate 920 is the sameas that of FIG. 83A and 83B, and the NAND gate 920′ is the same as thatof FIGS. 76 and 77. The inverter 906 is a known one. The operation ofthe decoder circuit of FIGS. 85A and 85B is the same as that of thedecoder circuit of FIG. 80A, and therefore, will not be explained again.Note, the transistors TRP906 and TRN906 constituting the inverter 906may be formed of larger size transistors than the other transistors (forexample, transistors TRP91, TRP91 and TRN93, TRN9n) to adequately passtransient currents caused by parasitic capacitances, and the like.Further, as described with reference to FIGS. 80A and 80B, when thedecoders (DEC1 to DECn) are divided into a plurality of groups (DEC1 toDECk, DECk+1 to DECn) and a plurality of inverters (906 a and 906 b) areprovided for the divided groups, the transistors (TRP906, TRN906)constituting each of the inverters can be formed in a smaller size thanthat of the inverter 906 provided for all of the decoders (DEC1 toDECn).

FIGS. 86A and 86B show the gates and transistors of the decoder circuitof FIG. 81A.

The decoder circuit of FIGS. 86A and 86B resembles that of FIGS. 85A and85B and differs from those of FIGS. 83A, 83B and 84A, 84B in that eachdecoder of the decoder circuit is formed of NAND gates 920 and 920′. TheNAND gate 920 is identical to that of FIGS. 83A and 83B, and the NANDgate 920′ is identical to that of FIGS. 76 and 77. The inverter 906 is aknown one. The operation of the decoder circuit of FIGS. 86A and 86B isthe same as that of the decoder circuit of FIG. 81A, and therefore, willnot be explained again. Note, the transistors TRP906 and TRN906constituting the inverter 906 may be formed as larger size transistorsthan the other transistors to adequately pass transient currents causedby parasitic capacitances, and the like. Further, as described withreference to FIGS. 81A and 81B, when the decoders (dec1 to decn) aredivided into a plurality of groups (dec1 to deck, deck+1 to decn) and aplurality of inverters (906 a and 906 b) are provided for the dividedgroups, the transistors (TRP906, TRN906) constituting each of theinverters can be formed of a smaller size than that of the inverter 906provided for all of the decoders (dec1 to decn).

Any one of the decoder circuits according to the ninth aspect of thepresent invention achieves a word line full selection function, a wordline nonselection function, a bit line full selection function, and abit line nonselection function. The ninth aspect of the presentinvention realizes these functions only by providing a second powersource for the decoder circuit involving a plurality of decoders. Thesecond power source has a switch including an inverter for selecting oneof at least two power source voltages. The ninth aspect of the presentinvention realizes these functions without increasing the size of thedecoder circuit. This decoder circuit, therefore, is appropriate for anintegrated semiconductor memory device.

FIG. 87 shows a power supply circuit for the decoder used in asemiconductor memory device according to the ninth aspect of the presentinvention. In FIG. 87, a reference 905′ denotes a second power sourcecorresponding to the inverter 906 shown in, for example, FIGS. 85A and85B, a reference EE denotes an erase control signal, and furtherreferences 920 and 920′ denote NAND gates which are the same as those ofFIGS. 85A and 85B. Note, an output of the power source 905′ is changedto a specific negative voltage (for example, −10 volts), when an eraseoperation is carried out.

As shown in FIG. 87, the power source 905′ comprises p-channel type MOStransistors TRP951, TRP952, TRP953, TRP954, TRP955, and TRP956, andn-channel type MOS transistors TRN951, TRN952, TRN953, TRN954, TRN955,and TRN956. Note, the erase control signal EE is at a high level H whenread or write operations are carried out, and the erase control signalEE is at a low level L when an erase operation is carried out. Further,the transistor (transfer gate) TRP954 is used to cut a negative voltage,and the transistors TRN954, TRN955, and TRN956 are formed on a triplewell configuration. Further, source voltages V_(BB) of the transistorsTRN954, TRN955, and TRN956 can be specified as a normal low voltage(ground voltage) Vss in the read or write operations. In addition, theoutput voltage of the power source 905′ is applied to the otherdecoders. In this embodiment, an address signal ADD indicating aspecific sector is applied to the NAND gates (predecoder) 920, theoutput voltage of the power source 905′ is applied to the decodersincluded in the specific sector.

As described above, the decoder circuit of the ninth aspect of thepresent invention can be applied to a flash memory for carrying out anerase operation (electrical and collective erase operation) thereof.

In the above embodiments, the decoder circuit achieves a word line fullselection function, a word line nonselection function, a bit line fullselection function, and a bit line nonselection function, in a testmode, but the decoder circuit of the ninth aspect of the presentinvention can also be applied to a flash memory having a collectiveerasing operation. Namely, in the erasing operation of the flash memory,all of the word lines must be collectively changed to a specificnegative voltage, e.g., −10 volts, and the circuit configuration of FIG.87 realizes this erase operation of the flash memory.

Recently, high function semiconductor devices (semiconductor memorydevices) such as flash memories have been driven with low voltage, andsome semiconductor memory devices have even driven with a negativevoltage. When driving semiconductor memory devices of conventionalspecifications with such low or negative voltage, the decoder circuitaccording to the ninth aspect of the present invention is advantageousfor switching voltages from one to another.

When a semiconductor memory device carries out various tests, a normalvoltage must be switched to a higher voltage than the normal voltage toset a test mode. If such a high voltage is applied to the semiconductordevice by mistake, the test mode will unexpectedly start to causemalfunctions and destroy data. Therefore, the high voltage (extremelyhigh voltage: for example, 12 volts) for starting the test mode in asemiconductor memory device is usually extremely high compared with avoltage (normal high voltage: for example, 5 volts) for normaloperation.

FIG. 88 shows a detector 930 for detecting the extremely high voltagefor starting the test mode.

A p-channel type MOS transistor TRP23 and an n-channel type MOStransistor TRN23 are connected to each other in series. A node N23between the transistors TRP23 and TRN23 is connected to a buffer BF23,which is connected to an output end TEST. The output end TEST isconnected to a test circuit, and gates of the transistors TRP23 andTRN23 are connected to a high voltage supply source Vcc. A source of thetransistor TRP23 is connected to a power source VIN, which provides anextremely high voltage much higher than the high voltage Vcc. A sourceof the transistor TRN23 is connected to a low voltage supply source Vssor to ground. The high voltage detector 930 causes no problems if thehigh voltage supply source Vcc is stable. When the high voltage supplysource Vcc is unstable, for example, at activation, the problemmentioned before occurs.

Referring to FIG. 89, the high voltage Vcc is applied to thesemiconductor device (semiconductor memory device) at time to and isstabilized after time t₁. A pulse P of the extremely high voltage VINhigher than the voltage Vcc is applied at time t₂. Then, a sourcevoltage of the transistor TRP23 becomes higher than a gate voltagethereof. If the difference between the source and gate voltages exceedsa threshold voltage of the transistor TRP23, the transistor is turnedON, and the terminal TEST provides a test signal T to start apredetermined test mode.

The high voltage Vcc is unstable during a period between t₀ and t₁. Ifthe voltage VIN rises quickly during this period and if a potentialdifference between the voltages VIN and Vcc exceeds, at time t₁, thethreshold voltage of the transistor TRP23, the terminal TEST willprovide a test signal TE to unexpectedly start the test mode. Thisresults in a malfunction.

This problem is usually caused when the voltage VIN is applied beforethe application of the high voltage Vcc.

FIG. 90 shows a test mode signal detector of a semiconductor device(semiconductor memory device) according to the ninth aspect of thepresent invention.

The detector 930 has a first detector section K1, a second detectorsection K2, and an operation circuit E1. The first detector section K1has a p-channel type MOS T81 and an n-channel type MOS transistor T82,and these transistors T81 and T82 are connected to each other in series.A node between the transistors T81 and T82 forms a first output end N81of the first detector section K1. Gates of the transistors T81 and T82are connected to a high voltage supply source Vcc, and a source of thetransistor T81 is connected to an external input terminal VIN. A sourceof the transistor T82 is connected to a low voltage supply source Vss orto a ground GND.

The second detector section K2 has a p-channel type MOS transistor T83and an n-channel type MOS transistor T84, and the transistors T83 andT84 are connected to each other in series. A node between thetransistors T83 and T84 forms a second output end N82 of the seconddetector section K2. Gates of the transistors T83 and T84 are connectedto the high voltage supply source Vcc. A source of the transistor T83 isconnected to the external input terminal VIN, and a source of thetransistor T84 is connected to the low voltage supply source Vss or tothe ground GND. The operation circuit E1 carries out a logical operationaccording to output signals from the detector sections K1 and K2, toprovide a test signal TEST.

This detector effectively prevents application of an unexpected testsignal, with no regard to the order of application of voltages. Thedetector detects the extremely high voltage VIN only on an expectedoccasion to start a test mode.

In FIG. 90, the supply source of the high voltage Vcc may be an internalpower source or an external power source. The external input terminalVIN provides the test mode starting voltage (extremely high voltage)that is higher than the high voltage Vcc. The operation circuit E1 maybe a NAND gate. An output end of the circuit E1 may have an inverterINV, if necessary.

During a normal state with the high voltage Vcc being stable, thetransistor T83 is ON, and the transistor T84 is OFF. Accordingly, theoutput end N82 of the detector section K2 provides a signal of highlevel H, and therefore, the test terminal TEST provides an output signalof the output terminal N81 of the detector section K1, thereby securingthe same operation as the conventional circuit.

When the high voltage Vcc is applied, there is a risk that the outputend N81 of the detector section K1 may provide a signal of high level Hby mistake. When the extremely high voltage VIN is higher than thevoltage Vcc and a voltage difference between them exceeds the thresholdvoltage of the transistor T81, the transistor T83 is weakly turned ON oris turned OFF to turn ON the transistor T84. As a result, the output endN82 of the second detector section K2 provides a signal of low level L,and therefore, the test terminal TEST is maintained at low level L.

Accordingly, as shown in FIG. 91, the embodiment of the ninth aspect ofthe present invention completely prevents the unexpected start of thetest mode.

FIG. 92 shows a decoder circuit shown in FIG. 85B, using the detectors930 shown in FIG. 90.

As shown in FIG. 92, two detectors 930 a and 930 b are provided forapplying signals An and AL. Namely, the detector 930 a receives anaddress signal An through a terminal (An), and when a voltage level ofthe terminal An is at an extremely high voltage (for example, 12 volts)higher than a normal high voltage (for example, 5 volts) for starting atest mode in a semiconductor memory device, a test signal TEST of a highlevel H is output therefrom. Note, when the test signal TEST is at ahigh level H, the decoder circuit 902 (901) carried out a test mode offull selection or nonselection functions of word or bit lines. Further,the detector 930 b receives a signal TT through a terminal (TT), andwhen a voltage level of the terminal TT is at an extremely high voltagehigher than a normal high voltage, the decoder circuit 901 (decoder 902)is set to a full selection mode. On the other hand, when a voltage levelof the terminal TT is at a low voltage, the decoder circuit 901 (decoder902) is set to a nonselection mode.

Namely, when the test signal TEST is at a high level H, a signal An isat a low level L. In this state, when a control signal AL is at a lowlevel L, a full selection mode is set in a semiconductor memory device(for example, EEPROM, flash memory) by the decoder circuit 901. On theother hand, when the control signal AL is at a high level H, anonselection mode is set in the semiconductor memory device by thedecoder circuit 901. Note, in the normal operation (normal mode) of thesemiconductor memory device, the test signal TEST is at a low level L,that is the voltage level of the terminal An is at a normal voltage (forexample, 0 volts to 5 volts).

As described above, the detectors can be provided for setting fullselection or non selection function (test mode) in the semiconductormemory device.

As explained above, a semiconductor memory device according to the firstaspect of the present invention effectively employs redundant word linesand achieves stable write and verify operations, thereby improving theyield and performance of semiconductor memories.

A semiconductor memory device according to the second aspect of thepresent invention replaces defective real cells with redundant cellswithout increasing the size of a semiconductor memory device, therebyimproving the yield of large capacity semiconductor memories andreducing the cost thereof.

A semiconductor memory device according to the third aspect of thepresent invention carries out a delivery test of a semiconductor memorydevice with “n” rewrite operations at the maximum, taking into accountdeterioration due to an increase in the number of rewrite operations, tothereby guarantee the maximum rewrite operations N (N>n) for a user.

A semiconductor memory device according to the fourth aspect of thepresent invention provides a semiconductor memory device in which thethreshold voltage of a write voltage supply transistor has no influenceon a write drain voltage, so that data is correctly written even with alow write voltage.

A semiconductor memory device according to the fifth aspect of thepresent invention correctly reads data even if there are overerased celltransistors.

A semiconductor memory device according to the sixth aspect of thepresent invention saves overerased cell transistors and correctly readsdata.

A semiconductor memory device according to the seventh aspect of thepresent invention simultaneously erases blocks of memory cells andeasily verifies the erasure.

A semiconductor memory device according to the eighth aspect of thepresent invention achieves a word line full selection function, a wordline nonselection function, a bit line full selection function, and abit line nonselection function. The eighth aspect of the presentinvention achieves these functions only by providing a second powersource for the decoder circuit containing a plurality of decoders. Thesecond power source circuit has a switch including an inverter forselecting one of at least two power source voltages. The eighth aspectof the present invention realizes these functions without increasing thesize of the decoder circuit. This decoder circuit is appropriate for anintegrated semiconductor device.

A semiconductor memory device according to the nineth aspect of thepresent invention employs two power sources but is as easy to handle asif it had only a single power source. This memory requires no step-upcircuit for the power sources and works with low voltages.

Many different embodiments of the present invention may be constructedwithout departing from the spirit and scope of the present invention,and it should be understood that the present invention is not limited tothe specific embodiments described in this specification, except asdefined in the appended claims.

What is claimed is:
 1. A semiconductor memory device comprising: aplurality of word lines; a plurality of bit lines; a memory cell arrayincluding a plurality of memory cells each formed of a MIS transistordisposed at each intersection of said word lines and said bit lines, athreshold voltage of said MIS transistor being externally electricallycontrollable according to charges to be injected to a floating gatethereof, and the floating gates of said MIS transistors beingsimultaneously discharged to collectively erase said memory cells; afirst power source for applying a normal selection voltage to a selectedword line to select memory cells connected to said word line, whenreading data; and a second power source for establishing an unselectedstate on unselected word lines including memory cells that have beenovererased by said collective erasing, when reading data.
 2. Asemiconductor memory device as claimed in claim 1, wherein said memorycells are formed of enhancement n-channel type MIS transistors, saidfirst power source is a positive voltage source for generating a normalpositive voltage, and said second power source is a negative voltagesource for generating a predetermined negative voltage for cutting OFFsaid overerased n-channel type MIS transistors that function asdepletion transistors due to said overerasing.
 3. A semiconductor memorydevice as claimed in claim 1, wherein said semiconductor memory deviceis constituted by a flash memory.